The Underground Cave is dark and damp. There is a huge, deep lake in the cave. Stalactites hang from the ceiling and stalactites grow on the floor. The land in the cave has a grayish-purple color.
Jake and the Banana Man fell into the cave when the rocket crashed into the ground. Finn went after them to save Jake. They all fell into the lake. Jake had to save Finn from drowning, but the Banana Man swam out on his own. This exhausted him, and he collapsed on the shore. While Banana Man was passed out, Finn chased Jake around the cave. Finn and Jake eventually got out of the cave. It is unknown if the Banana Man ever got out.
It is apparent that Finn, Jake, and the Banana Man were not the first to be in the cave. Embedded in one of the stalagmites in the background are the ruins of some sort of sign.
Stalagmites formed easily due to the cave's dampness. They are the same color of gray-purple as the land around them. They range in size from very tiny to bigger than Finn. The cave's main feature is its lake. The lake is so deep and dark that is resembles outer space. Banana Man's rocket sunk into the lake. In Jake's Croak Dream, Jake drowned in the lake, but it was Finn who really almost drowned in the lake.