The 8th-level spell Clone has a mention of a mud-filled cyst in the ground in its spell components:
a vessel worth at least 2,000 gp that has a sealable lid and is large enough to hold a Medium creature, such as a huge urn, coffin, mud-filled cyst in the ground, or crystal container filled with salt water
What is a "mud-filled cyst in the ground"? Not being a native english speaker, I've been searching online; the common meaning of the word cyst is usually some abnormal growth. Alternatively, a cyst can be some sort of bladder or sac.
This spell mentions specifically that it has to be in the ground and mud-filled, which makes me believe that it's based on some lore or reference. Anybody here any clues on what it is or how to find or construct it in a D&D setting?