Antlion Pits are hidden dungeons found solely in Volcanic Deserts. They are exceptionally hard to find for the inexperienced Erebus adventurer, or for those without keen eyesight.
Most often, a player will find an Antlion Pit by falling into it. This is due to the fact that the roof is made of Ghost Sand, which allows any entity to pass through unhindered. Hint: Ghost Sand lacks the white specks in the texture that normal Sand has. If you're sharp enough, you'll spot it. Look for 3x3-5x5 patches of it on fairly flat terrain.
At the bottom of the pit lies an Antlion Trove Guardian on top of a Chest filled with Erebus dungeon-tier loot.