The lobster trap was originally designed by the legendary gnomish trap engineer Erwil Snogglesnoutbottom to guard the entrance to the kings vaults. It consists of a room floored with an iron grid suspended some inches above a shallow body of water. The entrance door is set with a spring, causing it to close by itself unless impeded. Upon closing, the door triggers the trap. The flooring underneath the water starts to heat up, quickly filling the room with steam.
On the first round after triggering the trapped room becomes uncomfortable. On the secound round, any creatures trapped inside will start taking damage (1d6) as they are being steam boiled alive. The amount of damage increases each round (2d6 on the third round, then 4d6 each round thereafter). In addition, some dungeon masters may enjoy making players roll appropriate saving rolls to avoid claustrophobic panic taking a hold of their minds, rendering them unable to act.
The original lobster trap had no way of getting out once the trap had sprung - it could only be avoided by keeping the door from closing while crossing the room. Since then, many versions of the trap has been constructed - some with various ways of letting unintentional victims get out before being cooked. Some versions are made with the keyhole accessible from the inside, allowing someone with a key (or adventures with lockpicks) to open the door. Other versions have a hatch in the roof supplying an exit for anyone with a grappling hook and a length of rope.
The original trap had two exits, but versions have also been made with only one exit and some kind of bait at the other end of the room, luring foolhardy explorers to a hot, steamy doom.