Have you ever swum in the warm water of a lake in summer and found when treading water that your feet suddenly became uncomfortably cold? If so, you experienced something that limnologists, experts on lakes, describe by the rather splendid and poetic-sounding pair epilimnion and hypolimnion.
When the sun heats a smallish body of water, the topmost layer of water warms up, but because warm water is less dense than cold, it stays on top. That top area is the epilimnion. The cold water below it, which may not warm up much during the summer if the lake is at all deep, is the hypolimnion.
The root of both epilimnion and hypolimnion is the classical Greek limnion, the diminutive of limne, a lake. Limnologist and the subject of study, limnology, are very closely related — they derive from limne. Epi- is Greek for upon or above, while hypo- is from Greek hupo, under.
The epilimnion and hypolimnion are separated by a thinnish layer where the temperature drops quickly. You might guess this is sometimes called the metalimnion (Greek meta-, with or across), though it’s commonly referred to as the thermocline.
Most examples of epilimnion are in scientific contexts, though it also crops up very occasionally in SF:
The brown sphere was spotted after some days by a prowling ameba, quiescent in the eternal winter of the bottom. Down there the temperature was always an even 4°, no matter what the season, but it was unheard of that a spore should be found there while the high epilimnion was still warm and rich in oxygen.
Surface Tension, by James Blish, 1952.