Speleothems

From Muppet Wiki

Speleothems are cave formations that can be produced in the form of stalactites, which hang from the ceiling, and stalagmites, which rise from the ground.

A set of two stalactites and four stalagmites appear in a sketch in episode 307 of The Muppet Show, in which one stalagmite complains of a toothache while the rest of the speleothems echo him. After a series of jokes about the voices that the central stalagmite hears, the sketch ends by zooming out to reveal the first set of speleothems to be the teeth of a second set of speleothems, which, in suggesting an infinite regression, begin to repeat the sketch verbatim.

In episode 108 of Muppets Tonight, a set of four differently designed stalactites can be seen on the set of Jason Alexander and Gonzo's closing number, "Bats: the Musical."