I won’t apologize.
Summary: A still-functioning underground ruin from the Age of Strife lies opened in the wilderness. However, it is just an ancient deathtrap, created by some psychotic mechanical genius after the nuclear war, probably under the influence of Chaos. Upon entry, it fills itself with dirt and buries its inhabitants alive, then waits for them to expire before reclaiming the earth to await another unwary traveller.
Discovery: The entrance to the deathtrap is a large doorway about four meters tall and two across carved into the hillside. The door itself has retracted into the ground. A wooden sign has been nailed to a nearby tree. It says “KEEP AWAY” in Low Gothic. Examination of the sign with an Ordinary(+10) Awareness check reveals that it can’t be more than a few years old.
Exploration: The ruins beyond the doorway can be discerned as clearly ancient with a Challenging(+0) Forbidden Lore (Archaeotech) check. A Trade (Technomat) or Trade (Mason) check can also be made to identify the ruins, however no matter the results of the check, these Trade skills cannot be used to identify their construction. However, that is itself information, i.e. even with five degrees of success there is no result, so clearly the technology and/or style of architecture must be far, far outside the Imperial norm, either extremely ancient or xeno in origin.
Past the doorway are stairs that lead down a dozen meters into the earth. Characters descending the stairs or examining the way down from outside make an automatic Hard(-20) Awareness test, and notice chutes along the sides of the stairway with nearly seamless covers. The chutes can presumably open up to provide access to someplace or allow something to be poured down the stairs for some reason. The chute covers are AP8, and if destroyed will just release the stored earth to flood the chamber, although this will not cause the door to attempt to shut.
The stairs lead to a round, empty chamber about five meters across with a mosaic of a grinning skull paved across the floor. The skull is a pressure plate which will trigger the trap when stepped on. It takes up about two-thirds of the floor, so unless all characters specifically declare they are avoiding stepping on it, someone will step on it sooner rather than later. At that point, the door at the entrance will jolt three meters up before the decaying mechanisms jam. The skull pressure plate can be identified with a Very Hard(-30) Awareness test made automatically on entering the chamber to notice the seams around the skull, or if the skull is intentionally examined, a Challenging(+0) Archaeotech or Tech Use check or a Difficult(-10) Trade (Technomat) check. There is no way to determine what will happen if the pressure plate is stepped on without taking the whole chamber apart to examine it (see the checks for repairing/sabotaging the chamber in Reward for details on that), but if the characters have spotted the trap and have the basic survival instincts not to step on a hidden pressure plate of unknown function, they have basically solved the encounter.
Confrontation: As soon as the door has attempted to shut itself, the chutes open and begin pouring earth down into the chamber. It takes five rounds to fill up completely, at which point the mechanism will do nothing for an hour and then, when its victims are hopefully well and truly suffocated, the skull opens up to reclaim the earth, conveying it back up into the chutes along the wall. After this happens, the door opens again.
On the first round, the dirt will already have created a significant obstacle at the base of the stairs, but most of the chamber will still be empty. Characters may attempt to climb over the mound and up the stairs to the door. This requires an Ordinary(+10) Athletics check to get over the rapidly accumulating mound of dirt, and then a Hard(-20) Athletics check to get up the stairways against the current of descending dirt. If they fail, they will take 1d10 impact damage (use the Athletics check to determine hit location) to which armour does not apply, though Toughness and supernatural resilience still do, and begin the next round prone and being suffocated underneath the dirt that’s piling up at the base of the stairs. The chutes open up about half a meter past the door, so there is enough room for two average size characters to stand above the current, where they can wait out the trap until it reaches round five. Alternatively, they can try and clamber up the three meters to the door. This is a Difficult(-10) Athletics check, and if failed, the characters will fall back into the dirt stream and be pushed down the stairs. They suffer 1d10+4 impact damage (use the failed Athletics check to determine hit location) to which armour does not apply, though Toughness bonus and supernatural resilience still do, and also end up prone and suffocating at the base of the stairs.
On the second round, the dirt mound at the base of the chamber begins to collapse under the pressure of the chutes constantly shooting out more and more of it down the stairs, and the dirt in the chamber grows to about ankle height and the entrance to the chamber is now completely blocked and requires a Difficult(-10) Athletics check to climb through (including for characters who were flung to the base of the stairs by the strength of the current in the last round, as the current has carried them all the way into the chamber), in addition to the Hard(-20) check required to get up to the top of the stairs. By the third round, dirt has piled up to waist height at the far end of the chamber and is nearly to the ceiling at the entrance. Attempting to escape by climbing out now requires an Arduous(-40) check to climb onto the stairs in addition to the Hard(-20) check to get to the top of the stairs. In the fourth round, the entire lower chamber is filled and any character still there will begin to suffocate. In the fifth round, the stairs will also be entirely filled. Anyone attempting to climb up them must now succeed on an Arduous(-40) Athletics check and will be suffocating each round, however because the current has ceased, failure simply means no further progress is made this round, rather than pushing characters back down into the main chamber. The small safe zone above the chutes still exists, though it is ankle-deep in dirt sprayed out the sides of the chutes while in operation, and two characters can still stand there safely, fully able to breathe thanks to the door having stopped one meter short of sealing up airtight, and can either wait for the door to retract in an hour or attempt to climb out. Failures to attempt climbing out will land in soft, packed earth and will not be blasted down the stairs, so there is no penalty to failing, which means anyone who wants to climb out will eventually succeed.
Any attempt to blast through the door is going to do far more harm to the characters than the dirt. The dirt has AP32 and anything that does extra damage to structures will not do extra damage to a giant pile of dirt. However, one thing a clever and/or lucky character may try is to blast through the floor, where the dirt reclamation system is positioned. The floor is AP16 and at least ten points of damage must get through to create a large enough hole to rapidly drain the dirt. If ten points of damage are dealt to the floor, the dirt reclamation system below will consume the dirt at a fast enough rate to prevent the chamber from filling up completely. Characters will be able to keep their heads above the dirt and breathe and the dirt reclamation system will slowly clear away the filling in the hallway provided that no one decides to try and make a break for the surface. Getting onto the stairs and then up the stairs still requires two separate Arduous(-40) Athletics checks, and failure still means the character is trapped beneath the dirt and suffocating, so attempting to leave early can be fatal, but otherwise the characters just need to wait an hour. They won’t run out of oxygen, because the dirt drains quickly enough for an air passage to open up to the door (still only partly closed) before they run out.
Rewards: Assuming the characters don’t destroy the trap while escaping it, it will work the same on anyone who steps on the skull. A Hard(-20) Tech Use check will even be able to identify the repairs needed on the door, and a Common(+10) Requisition check will get the parts needed. Once parts are obtained, any character who succeeded on the Tech Use check to identify how to repair the door will be able to actually perform the repairs in about an hour with no further check needed. The same Hard(-20) Tech Use check can be used to disable the chutes or fiddle with the reset timer on the skull, which can be used to disable the chutes completely and make the door reopen immediately after weight is removed from the skull. At this point, a heavy enough rock to trigger the skull can be used to keep the door shut, removed when the characters want to leave, and the place can be used as a small base camp, outpost, or storage cache. Disabling the chute and fiddling with the timer requires no Requisition test, because no parts need to be swapped out, just modified or, in the case of the chutes, removed (the removed part from the chutes will not help fix the door, however).