It appears as a regular ant hill entrance.
The interior layout is almost identical to the ant mound in Fallout: New Vegas. It features several long, narrow tunnels that are dotted with cave fungi, among other things. At the end of the tunnels, there is one round room with two corpses that have little to no valuable loot on them.
The interior is small and holds little of interest, with a few hostile ants, waste piles which may contain ant nectar, and a few lootable corpses with common items.
The musty caverns appear only in Fallout 3.