A Pit Monster (also occasionally known as a "Sand Monster" or "Sand Trap") is a type of alien hazard appearing in the levels Unearthly Excavation and Penumbra.
Appearance
The Pit Monster appears as a large, pulsating pink circle lined with several large teeth protruding from its sides. Surrounding the top of this circle is a light-brown ring of lumpy flesh, similar in appearance to dirty stones.
Behavior
Pit Monsters make an intermittent growling sound until filled with sand.
Pit Monsters will consume almost all items that fall into them. This can be used as a disposal method, convenient when the player is far from the Incinerator. However, each time they are "fed", there is a chance they will regurgitate one or more of the items thrown in, plus a random amount of viscera (both gibs and whole limbs). They will also spray out green digestive goo in a wide radius, creating extra mess for the player to clean up.
There are a few items, including the Shovel, Laser Welder, Big Banger Supernova, and the Alien Artifact that Pit Monsters will refuse to eat. A shovel will immediately bounce back out of the Pit Monster, presumably so the player cannot lose the only tool that can fill the pits with sand. The Big Banger Supernova (while playing music) will bounce around inside the monster for a time before being spit back out.
A player who falls into a Pit Monster will be immediately killed. This is one of the few ways a player can die in the game. (Note that there is an achievement for being eaten by a Pit Monster.)
Normally, shovels are dropped if a player dies while carrying one. If the player dies from falling into a pit monster, however, the shovel may also be "eaten". Recovering the shovel is possible by throwing items into the Pit Monster until it regurgitates the shovel.
Filling with Sand
Pit Monsters must be filled in with sand using a shovel before punching out of the level in order to obtain a perfect score. Each correctly-placed shovel full of sand will cause the sand inside the Pit Monster to rise. If it doesn't, the sand was either not dumped in the right spot, or the Monster is completely buried. Once completely buried, Pit Monsters stop making sounds and become safe to walk over.
It takes approximately 40 scoops of sand to completely fill a Pit Monster. Sand can also be removed from Pit Monsters at any time, using the shovel.
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The view directly above a Pit Monster.
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The same Pit Monster filled in with sand.
Trivia
The center of a Pit Monster has no contact area, and simply consists of a trigger to remove any items placed into it, or to kill any player who falls into it.
Prior to a collision-area fix, the light-brown ring around a Pit Monster was not solid, meaning it was possible for items to become stuck inside them. Blood and other messes could lay underneath the Pit Monster without being visible to the player, but could still be detected with the sniffer. Players who were aware of this bug could often clean up blood or soot with the Mop or dislodge items stuck in this area with various tools.