Some background on who built this megadungeon. Well, I haven't decided yet. But I do know who built the gatehouse, not that the players will likely ever piece the information together. Megadungeons are mysterious places, that through their very nature attract dangerous monsters, devious traps, and untold riches. They are more than just holes in the ground - should you somehow clear a megadungeon from top to bottom of all creatures, loot all its treasure, disarm all its traps, they would slowly reappear. As nature abhors a vacuum, so it also abhors an empty megadungeon.
In the ancient, forgotten past, this megadungeon and its supernatural resources were discovered, and given the nature of the times, exploited by a megacorporation. The gatehouse portion of the first level was excavated by the megacorporation, to provide a safe haven for the scientists and explorers that it was sending into the depths. The men who ran that company had the insight to realize that what they found was more than just bits of treasure guarded by hideous monsters, but a place ruled by physical laws subtly different than those of the surface world. In this special place, they explored, experimented, and died. Sealed off for the passing millenia, it has now finally been reopened.
Parts of the gatehouse were used for training potential explorers. These live training exercises were potentially lethal. See below, an attempt to teach students of the perils of riddle-traps.
24. Subsurface Training Simulator
This room is unusual in the gatehouse, as the floor, walls, and ceiling are bare stone, uncoated with the white substance found in the other rooms and halls of the gatehouse. The ceiling, being stone, does not emit any light, and the room is dark.
The north wall of this room has a large protonium-metal chest sitting on a raised dais. To either side of the chest are two poles, with horizontal rings attached to the top. The poles, if examined, can be tilted to face the SW and SE corners of the room.
In the alcoves to the southwest and southeast are two crystal living statues (AC 4, HD 3, hp 17, 9, MV (30’), #AT 2, D 1-6/1-6, Save F 3, ML 11), one in each alcove. They resemble warriors in chainmail, holding swords. They are standing on stone pedestals – the pedestal in the southeast alcove inscribed with “In Darkness”, and the one to the southwest inscribed with “We Dwell”.
If the players try to open the chest, the living statues will attack until the players leave the room. The only way to prevent this is to place light sources on each of the two poles, and tilt them so they lean towards the statues. If this is done, the players can safely open the chest without fear of attack.
In the chest is a gold ingot, inscribed with the words “Simulated Subsurface Treasure”, worth 600 gp.