A speleothem was a type of geological formation found in underground caverns.
In the 22nd century, the planet Xantoras was renowned for having some of the most unique and largest collections of speleothems known.
Dr. Yolen, a Denobulan geologist for the Denobulan Science Academy, started studying speleothems in 2138. In 2152, he was able to begin research in an underground tunnel on Xantoras, but was forced to leave his work several weeks early because of a change in local government. (ENT: "The Breach")
In the final draft script of "The Breach", speleothems were defined as "beautiful, oddly shaped mineral deposits".