The Bottomless Gorge is a location on Third Earth. This huge ravine is purported to have no bottom.
In the episode Prank Call, Lion-O, WilyKat, and WilyKit swing across the Bottomless Gorge during their exploratory trip. WilyKit suspects that the gorge must have a bottom.
The Bottomless Gorge is most likely a nod to the Bottomless Chasm, a similar location that appeared in a number of episodes in the original 1980s ThunderCats cartoon show.