Pit Worm

Encountered in a sewage processing area in the lower levels of Black Mesa, this dangerous creature resembles an enormous caterpillar, and bears resemblances to the Gohma boss from the Zelda series. It makes bird-like vocalizations. Its segmented body supports a head dominated by a single large eye. This eye is the pit worm's primary weapon, capable of firing a searing energy beam at nearby enemies. If enemies get too close, the worm also has large claws to strike out with. If the worm is shot in the eye, it will recoil, shielding it with its claws for a few seconds before striking aggressively at the player.

The pit worm obstacle is similar to the tentacle sequence from Half-Life, in that the player cannot simply use his weapons to defeat the creature, but must use the surrounding environment. The process of destroying the pit worm requires the player to dissolve it by turning a valve and activating a gearbox, obvious references to Valve Software and Gearbox Software, the developers of Half-Life: Opposing Force.