The Great Tree. A giant tree, of an unknown species (most think its oak, but no one knows for sure), growing on some plane that is presumed to be inaccessible, though it may be possible to travel down through the twisting tunnels to the very bottom, and find an exit there. Each branch of the the tree is ended by a world, where the branch enters the world. And yes, the Great Tree was borrowed from Norse (for the uneducated: the Vikings) mythology. See 'Yggdrasil, the World Tree.' The Great Tree itself is impossibly old, impossibly smart, and knows about whatever goes on inside itself.
The Parts of the Tree:
The Branches: Each branch of the tree is a necessary part of that world. If the branch is chopped off, than the world's natural laws, and everything that bound it together, would eventually die. This would mean that physics would slowly stop happening, so that gravity wouldn't work, inertia would cease, etc. The sun(s) would, too, stop working, so that plants wouldn't get energy, and die. Everything that held the world together would wither and die. On another subject, the branch can enter the world anywhere. It can be a thousand feet beneath the sea. The chopping of the branch (and the killing of the world, and the genocide of the world's people) can be down by simply chopping down the tree that marks the location where the branch enters the world. The species of the tree can be any, but the tree will lack roots. The tree's trunk will, if digging occurs, simply go straight down. The tree will also be very, very old.. The wall between the world and the branch is thinner here, making it easy to enter the Great Tree's branch (and from there the trunk). Through either force of will, the use of a spell, or a machine (in a sci-fi universe, people would have to study it for a bit, and then create a machine allowing access), a person can enter the trunk. This tree can be on top of the highest (or most monster-infested) mountain. It can be in the middle of a city. It can be somewhere in a mile-wide forest.
The Trunk: Riddling the trunk of the Great Tree is a great many completely circular passageways. These passageways are for the ant-like species to live in the Great Tree in a symbiotic relationship. The ants get a home, the Tree gets fertilizer (read ant-poop) and a defense system against intruders (read adventurers). The ants have split into several colonies, with a colony on each level of the tree. They will have a main base somewhere on the level, with a large central area for meetings, several sleeping quarters off the main base, a "hospital," the queen's room (which will be larger than a sleeping quarter, where 20-30 ants sleep), and a place for storing food. The ants have a human-like intelligence. They will kill any intruders they find, as they will kill any other colonies' ants that are raiding them. The various colonies are in a war with each of the other colonies. The tree provides a green food-resin for the ants, which is secreted from the passageways. As such, the ants will have foraging parties to collect the food-resin out constantly. The Tree, besides ants to guard it, also has its own ways of protecting itself from intruders. The passageways themselves are maze-like, which makes finding a branch/world difficult. The entrances to a branch is marked by a door, which leads to a long corridor that ends in the other world.
Possible Plot Hooks:
1. The tree needs to be pruned, because it lacks the energy to sustain all of its branches. Either some of the other branches go, or all of them go, and the universe collapses in on itself. So the party has to traverse the great tree, find a world, and destroy that branch/tree (and committing genocide in the process). But the problem is that those other worlds also know about the Tree, and its need to be pruned, and they sent their own bands...
2. The world is suffering under the weight of a disease contracted from stranger from another world (like the native peoples got smallpox from the Europeans). That other world has a medicine to stop the disease. And so the party must travel through the tree and acquire the medicine.
3. An explorer, who went into the Great Tree, found a land where gold is almost as common as dirt. Only the fact of how difficult going through the tree, getting the gold, and taking it back is stopping the gold prices from dropping too much. But an intrepid group could make an easy payday. All they have too do is fend off ants both ways, take care of the natives (through words or swords), and mine that gold.
4. (This isn't really a plot hook, just an idea) A now-dead adventurer previously found the location of the tree marking where the branch enters this world. He charted its location on his map, set out for a city to tell the world, and promptly got swallowed by a dragon. The dragon got the map (and the gold, magic items, meat, etc), went home, and stored it in its nicely organized treasure cave (just because its a monster does not mean it, with its apparently high intelligence, would lack simple organization skills and dump it all in a pile.