Half Life: Blue Shift Easter Egg - Chumtoad's Lair

Very soon after you arrive on Xen, you will enter a series of cramped, narrow tunnels full of headcrabs. Nearing the exit of this maze, you pass through a large cavern with an extremely deep pool at its centre.

At the very bottom of this pool, between a boulder and three Xen plants, a small crack in the wall of the cave can be seen emitting bubbles of air. After widening this hole, you can enter a small air pocket in a cave where the title "Chumtoad's Lair" appears on the screen. In the Lair, three chumtoads sit amongst Xen plants and snark nests, illuminated by Xen crystals. The creatures later vanish when approached.