Speleothems are cave features formed by minerals. The word speleothem comes from the Greek words spelaion meaning "cave" and thema meaning "deposit". The most well-known speleothems are stalactites and stalagmites. Stalactites grow down from the cave ceiling. Stalagmites grow up from the cave floor. It’s easy to remember which is which: Stalactites have a "T" for top and stalagmites have a "G" for ground.