The mound is what the PCs probably thought it was when they started digging, a barrow for some king, queen or chieftain of the distant past. It may well have been robbed already in which case the PCs will find some human bones and that is all. Or it might still contain treasures of old, such as Elric's Silver Hammer or the Iron Sorrow. If the GM is feeling unkind the amour of the dead king might be made of Orthacarium. Barrows often contain dangers such as a Barrow Dragon, an Urn Beast or a Barrow-Wight to attack the intruders. It could be that the barrow they are breaking into is one of the Barrows of The First Men which if they are doing so at the right time could be an adventure by itself.
2-Portal to another World
Under the earth a Stargate is buried;if the PCs find out how to activate it they have a portal that leads to another world, totally different to the one they are in. If they use it the GM can decide if it is a one way trip or not.
The PCs, after digging for a while, come across a sign bearing the international symbol for "Do Not Dig Here." If they ignore it, and PCs being PCs, they might well do, they come across a smooth metallic substance. If they dig around it with care, it will be proved to be a large pipe of some kind. If, however, they mistake it for the lid of a sarcophagus or a chest, and try and break it open, a crack opens and a jet of yellow-brown foul smelling sewage jets out and covers and PCs within six feet of the broken pipe. The PCs have dug up and broken the sewage pipe coming from the nearest big town. Until they can find a way to wash it off themselves and their clothes and weapons, innkeepers will be reluctant to serve them or let them hire rooms for the night due to the stink of it, and sneaking up on anyone or anything when not downwind will be impossible as the airborne stink of human waste will make it clear that the PCs are nearby.
4-Secret Military Base
Unknown to the PCs, what they are digging into is the entrance to a secret military fortress, and their actions will alert the gate guards who will attempt to place them under arrest and take them to see the colonel in command. If they submit peacefully and the region is at peace, they will most likely escape with a telling off and perhaps a small fine. If they submit peacefully but the region is at war, they will face a court-martial on a capital charge of spying. If they fight and kill the trained soldiers that are guarding this hidden gate, a whole company of professional soldiers will soon be on their trail with orders not to bother to try and take them alive. These are not lowly conscripts but hardened warriors who know the area and the PCs will find it hard to escape from them.
The PCs find that when they dig deep enough, the mound is full of weapons, mainly ordinary swords and spears,rifles, or lasers, depending on the time period that the GM's world is set in and if it is a fantasy or a sci-fi world. If a fantasy world there may well be a few magical weapons here such as The Sword of Fire , if a more tech heavy world the PCs might find one or more of The C-47 Thaumatech Rifle. As to why the weapons are there, it may be an armory from the distant past, in which case most of the weapons will be dull with rust at best and cannot be used at worst. Or it could be part of the secret military base from #4 in which case a squad of professional soldiers, one soldier per PC, will soon be looking for whoever broke into their arms dump and stole some of their weapons. And they won't be happy about it, to say the least.
6-Radioactive waste dump
As with #3, the PCs will soon encounter international symbols that warn against digging. If they ignore them, they will come across metal drums, and if they pull them out and break them they will encounter radioactive waste if in a sci-fi game or it's fantasy equivalent. How dangerous it is depends on how long it has been buried for and how unkind the GM is feeling. The effects can range from mild sickness and nausea to losing teeth and nails to severe illness resulting in death if not quickly healed in some way. Some things should really stay buried.
7-Buried wizard's tower
One of 30 Necromancers has done what most wizards like to do and built a tower. Since the majority of his spells are greatly frowned upon by the law and most people in general, and to hide from The Upright Society of Civic Wizards">The Upright Society of Civic Wizards , the police force and anyone else who might cause him trouble. Should the PCs break in they will find undead guardians and possibly booby traps awaiting them as they try and plunder the tower. But the Necromancer is not trying to kill them out of sheer badness; he is just defending his possessions and himself from intruders. Should they confront him he will point out that he was staying out of their way until they broke into his home and started robbing it and killing his undead pets.
Some things they might find on their way down...
Handy Hand- A human hand with a dishcloth, cleaning the floor. It is not meant for combat and if they attack it will just scamper out of their way.
Doggy Bones- Two dog versions of Ironbones that will clatter their teeth at the PCs in a soundless parody of barking before leaping to the attack. In their case, their bark is genuinely worse then their bite.
Rug of Doom- A large bearskin rug with teeth and claws that if stepped upon will attack; it has the strength of a living bear and must be slashed into pieces to disable it.
Family pictures- The Necromancer aged in his 20's with his arms around a pretty young girl, the love of his life.
A Scroll of mute that the Necromancer was keeping for when his in-laws were coming in a few days to visit. PCs being who they are, one, perhaps the PC mage or witch eager for more power will read it, trigger it on himself or herself and be unable to speak for an hour in real time or to cast any spells that require speech during that time.
8-Entrance to Nekron city 1666
The PCs can take this two ways, they can get into a fight straight away, which is what many PCs would do when faced with another race. More diplomatic PCs can talk with the Nekron, who know their language and if they have something good to trade they might even be able to talk their way into the Nekron city, that is a sight to behold. As long as they behave well when down there and obey the complex laws they have nothing to fear and can buy things down under that they could not buy Topside, such as tasty/magical mushrooms, glowing sticks of mori rock that won't ignite underground seams of gas, and certain kinds of weapons.
The PCs have discovered the secret hideout of a band of well-armed and aggressive brigands, two brigands per PC, with a low-level magic user amongst them who can cast heal wound on one of his fellow bandits per round and fireball at one PC per round. If they kill more then half the brigands, the rest will try and flee rather then fight on. If they kill or drive away all the brigands, they get to loot their weapons and money and they find a bound gagged nobleman that the brigands were holding as hostage. Assuming the PCs untie him, he will invite them to his father's court. At the very least they will get a small reward; if they hand back the money that the brigands stole they will be thought of as honest, will be more popular with everyone and will be offered quite well-paid jobs as bodyguards or City Guards. If they know court etiquette 4641 they will be even more popular at court and may be able to wangle something really big such as an armed escort for the next part of their journey or the gift or loan of a magical weapon.
The PCs, after a bit of digging, find a large number of human skeletons buried in the mound, but no grave goods of money apart from maybe a coin or two. Unknown to them, a plague swept through this region decades ago and the bodies were buried in what became this mound. Even though such a long time has passed since then the germs are still infectious and one of two things is likely to happen. Either one or more of the PCs will come down with the disease and face serious illness and possibly even death, in which case symptoms will break out within hours. Or one or more of the PCs are carriers of the disease and do not suffer from it themselves but spread it to all who they interact closely with, leaving a trail of illness and death behind them everywhere they go and becoming very unpopular. If news gets ahead of them cities will close their gates to them and improvised militias will try and stop them entering villages.
The disease could be bubonic plague or something of that kind, or it could be something even more dangerous like the 1812 Omen plague.
An Elemental (the GM chooses the type) has been magically imprisoned in this mound, and the digging of the PCs breaks the spell and sets it free. Depending on how sane and how angry it is, it will either grant the PCs one wish within reason for setting them free, or will attack them, or will just start destroying everything it sees in a crazy rage. It can only be harmed by certain spells and by magical weapons. It need not be one of the four classical elements, it could be one of many kinds 4181. If it just goes off in a rage and the PCs do nothing about it, they will face trouble if people find out they set it free, anything from angry vigilante posses to arrest and trial by the local law enforcement.
The PCs seem to have really hit the jackpot with this one. They find gold and jewels, plenty of them, and a little deeper down they find things that look like eggs. Huge eggs the size of watermelons. Suddenly a roar is heard, and in the distance a massive, fully grown dragon is seen flying rapidly towards the PCs. If they are stupid enough to stand and fight, the dragon will snap them up, but there are some caves not far away that are too small for the dragon to enter, which will lead to a new part of the roleplay, played underground. If they have smashed any of the eggs, they will have made a lifelong enemy of the dragon for slaying one or more of it's unborn children and it will search for them for years.
Sometimes a mound is just a mound that was formed by the earth itself. Digging this up will be a waste of time as there is nothing but dirt and more dirt inside.
The mound holds the compost for the local farm, and the PCs risk getting covered in it and stinking to high heaven with the same problems caused as in #3.
There are rich pickings here, Funeral Gold and Grave Silver to be found on the embalmed bodies of the dead dwarves within this tomb. However, the tomb has a guardian, a fearsome and ferocious Dwarven Beard Golem and should the PCs defeat it and be seen by nearby dwarves with their stolen loot, the vengeance meted out by the entire local clan of dwarves will come down on their heads. Dwarves hate with a passion anyone who would defile and rob their tombs.
The PCs dig in through the roof of the house of a Hobbit. Nice PCs will say sorry and repair the damage. Nasty PCs will walk away and leave the horrified Hobbit to try and repair the damage himself. Very nasty PCs will murder the Hobbit for complaining and loot his house, and gain the dislike of the *little folk* which will try and damage their campaign in subtle ways. A fairy, for example might lure the PCs into a swamp, or the PCs may find themselves under a curse whose effects grow worse and worse with every day.
17-No Digging Allowed
By chance the local law enforcement catches the PCs in the act, orders them to stop and fines them a small sum, just enough to annoy them. They can pay the fine and leave the area, pay it , leave, sneak back and dig again (in which case choose again to see what is in the mound) or they can fight and maybe kill the local law enforcement and face a posse sent out after their heads.
There is a seam of flammable gas in the mound and if it is ignited by a lit torch, a spell, a lit pipe or just a spark from a spade of pickaxes it will ignite, burning the PCs nearest to it and smashing a hole in the mound.
The entrance to a disused mine is revealed; it is up to the GM if it still contains anything of value and how dangerous entering it would be. It does not have to have any monsters, undead or deliberate booby-traps to be dangerous; deep vertical mine shafts and an unstable roof prone to cave-ins is just as deadly to the unwary who underestimate them.
The mound is full of the waste from coal mines, of very little value except possibly as fuel for a cooking fire.
The PCs find a buried Droid. If in a fantasy world the eons of time have most likely reduced it to a rusting piece of junk, if in a sci-fi world it is up to the GM if the Droid is a defense droid that attacks them, can be used by the PCs either for battle, for transport needs or both, or has been deliberately rendered beyond use before being buried.
22-Broken remains of starship
If in a fantasy world this will be from a very distant time indeed;even in a sci-fi world it is little better then rusting remains although it might be possible to salvage something of use from it.
23-Buried Holy Place of Old
The PCs uncover an altar and can sense a holy aura coming from it. It may be a long forgotten god or goddess that was once worshipped here, alternately it could be an altar to an evil cult like The Children of Ma-0 or The Cultus of Vautu. If the latter it was either buried by the cult itself to escape discovery or by law enforcement/ holy paladins/the Inquisition. If the former and there are cult members still present in the region they will think that the PCs are trying to expose their activities; if the latter, the PCs may well themselves be mistaken for cult members and treated accordingly until they either prove their innocence or manage to escape from their pursuers. Or there may be a third reason; an intolerant religion has banned all the other religions in it's territory and buried or outright destroyed their holy sites.
All the PCs will find here is fish bones and the like but at least they won't be left stinking as a result of their labors.
A mighty geyser of boiling water bursts out, scalding the PCs if they are not careful. Depending on the GM and the magic level in the world, it may or may not have magical/healing powers. Or it could just mean that the PCs dug into and broke the nearest town's aqueduct and cut off it's supply of hot water until the pipe is fixed.
A century ago there was a war here and the whole area was a minefield. After the war was over the mines were dug up with care so that they would not blow up farmers, travelers and other random people, and were placed in a shallow pit and a mound of earth was placed above them, with warning signs and a high fence around it. As time went by the signs weathered away and fell apart and the fence was taken for other uses but the local people still know not to dig up that mound from what their parents told them. Not so the PCs; they risk either setting off one of the mines and one of them getting badly hurt or killed, or worse still, setting off all the mines in a catastrophic chain reaction and being blown into pieces of shattered armour and small bloodstained pieces of flesh, and the locals will shake their heads at their stupidity.
27-Buried cave entrance
The PCs find a cave; it may be a small one or the entrance to an entire subterranean network of tunnels and caverns for them to explore.
The PCs find that they have dug into the den of a large, fierce and very angry bear and her two small cubs. As she has her cubs to protect, she will most likely fight them to the death. If they kill her they can take her cubs along as pets or to sell in the nearby town if they want to, but the cubs will soon be thirsty and will need milk to drink.
29-buried gallows and gibbet
The PCs find a metal cage with a dead skeleton dressed in rags within it; if they bother to dig any further they find the gallows that the cage is hanging from.
The PCs find an ornate marble pentagram and given that it was buried, it was buried for a good reason. It could be that if a PC puts a foot into the pentagram a demon appears and tries to kill him or her or drag him or her down to hell. It could transport the PC to another part of the country and the other PCs have to decide if they want to follow or not. Alternately it might only be dangerous if the PCs try and summon a demon within it;or it might be so secure that a demon can indeed be summoned safely within it if the PCs know the right spell to do so.