In a good LARP it is desirable to separate the players out individually at some stage, because they are more suggestible and afraid when they are alone. Some form of individual challenge is probably the best way of doing this. One possible setting for this is a cave system, and it is probably one of the easiest to recreate realistically, and one of the most perfect for playing on fears of claustrophobia and darkness.
Given an ordinary room, how does one make it appear to be a cavern? The answer is that one doesn’t. Take advantage of the fact that a cave is intrinsically a dark place, and you find that there is no need for elaborate decorations. So our first aim is to make the house as dark as it possibly can be. Cover windows up with curtains and shutters, failing that cardboard and sheets. Cut out as much light as possible. For this reason it is best to try such an adventure out at night time and during the winter months.
To add to the sense of disorientation effected by the darkness, spin your players around a little, walk them in circles and make them dizzy.
This gives us the first phobia, darkness and a sense of being lost. In a small house this may not last for long, but in LARP it is the first sensations which count: once you are afraid you remain afraid. To further the feeling of being in an alien environment it is often beneficial to run your adventures in a house unfamiliar to your players.
So what is our next phobia? Claustrophobia. To recreate the tiny constricted tunnels that riddle the earth, matresses come to our aid. Find a narrowish corridor or hallway and wedge a matress in horizontally and at about shin-height. This is for people to crawl beneath. Obviously they will not crawl under it unless they have to, so rest another matress on top of this leaning at 45 degrees to the vertical, supported by being tied to a bannister or leaning on a sloping roof. Large cardboard sheets would also suffice.
Piles of boulders are a danger in cave systems, so why not in LARP? Use cardboard boxes, stacked so that there is only a narrow way through, precariously so that they are likely to fall if someone is careless. These want to be relatively heavy (without being dangerously so), so fill them with shoes or cardboard.
And so for the third phobia…the dwellers in the caverns. Of course it is difficult to organise actual inhabitants, so instead we leave _traces_ of the inhabitants. Spiders’ webs can be particularly fine when made from sellotape and cellophane. Not only do they glisten slightly in any candlelight, they are sticky and someone walking into one in the dark would be most surprised.
Rustling noises can be accomplished by attaching chains of crumpled paper to string which is fed through a hook on a wall and operated at a distance.
On the subject of sound, recall that the primary sense available to a cave-explorer is hearing, and caves have distinctly different sonorous qualities to houses. This is difficult to remedy, but a cunningly concealed amplifier with a microphone hanging in the vicinity of the cave “entrance” can produce a feedback loop which mimics the correct quality of sound. This need not be reproduced throughout the entire cave system: remember once you have created a suspension of disbelief it will stay intact for quite a while, especially if the adventurer is alone.
If we are to go for full sensory immersion then a few fetid smells wouldn’t go amiss: leaving some tuna to go off for a couple of days and then placing it near a tape-player which emits the sound of running water or dripping will create a fairly good illusion of an underground water system. In addition, you could dampen the carpet beneath the player’s hands when they are crawling, provided the owner of the house doesn’t mind.
If you were to run such a session with a group, you could introduce more interesting tasks, such as absailing and climbing down stairs. A good roleplaying group would make good roleplaying out of this, while a bad group would probably just injure themselves.
Remember though that the suspension of disbelief won’t last indefinitely, and a house is only a finite size. It is best that a period of LARP speleology lasts no more than five or ten minutes per adventurer: any more and they will lose interest. Perhaps if you interspersed short caving sections with encounters in lighted rooms you could spin it out for longer, and a really good group of roleplayers (rather than mere adventurers) would probably be fit to explore as a group rather than individually: remaining in character would strengthen the feeling of realism and prolong the experience.
The most important warning of all I leave till last: remove all breakable items and hide them out of sight. Adventurers stumbling in the dark are liable to break things.