Speleothem
A speleothem was a type of geological formation found in underground caverns. 

In the 22nd century, the planet  was renowned for having among the largest collections of unique 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. ()

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Materials
Geology