This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The Caves of Mak'ala were a series of interconnecting underground caves on Trill. Deep within these caves were kilometers of interconnecting pools of a milk-like substance which was the natural environment of the Trill symbionts.
The symbionts would die unless they were in the milk pools or were joined after a certain amount of time. When living in the pools, they could communicate with one another via electrical pulses sent through the liquid. They bred and were born in the pools.
In 3189, Commander Michael Burnham and Adira Tal traveled to Trill in order to repair the connection between Adira and the Tal symbiont. With the assistance of Guardian Xi, the pair traveled to the pools, both eventually submerging themselves and connecting telepathically to work through blocked memories of Gray Tal's death. (DIS: "Forget Me Not")