A simple yet luring trap for those so foolish to be curious, gnomes beware. The trap begins at any door. Upon the opening of the door a deep dark pit presents itself on the other side about 3′ from the door, its top quickly sliding open. Extend the opening of the pit just enough to make a jump difficult, perhaps 15 to 20 feet. The pit is 20′ in depth, enough where those that fall can’t get out easily. Looking down the pit will yield nothing but darkness. On the sides there are crevasses and holds for one to hoist themselves out rather quickly. One might notice some larger recesses within the walls (search DC 15) (2E INT -2) near the bottom also.
Jumping the pit would be difficult (DC 20) (2E STR -4 or Jump -2), any failure leads the jumper to fall in for 2d6 damage.
Once a PC has fallen down the pit or smartly lowered himself down for a look see the surprise begins. A twisted web of natural forming tunnels lies at the same level of the pit, 20′ below. Two of the tunnel ends lead to the pit itself, very dark and hard to spot from above. Within the tunnels place an appropriate creature to make the PCs wish they had never gone down there or failed their jump. Personally I enjoy wights as a nice present. They come out with the entering of the unwary.
Now a clever party might make it across by magic, ingenuity, or sheer jumping power. Have no worries you as the DM are not beaten yet. At the end of this hallway lies another door, perhaps metal, that can not be opened unless the 1st is closed. A mechanical device locks the door. During the time the party is looking at the door for a way through, picking locks, bashing, searching, the wights in the pit are climbing out preventing any exit.
If your nice you can allow the farther door to be bashable or pickable but I wouldn’t. The trick here is to open the 1st door, walk in, and close it behind you as you stand on the ledge. When the 1st door closes the pit closes and the 2nd door unlocks. Thus the undetectable trap.