@mrpopstar
More thinking out loud.
Living under the surface is mythologically accurate for Celtic fey. The fey are the places where the land is fertile. They are land spirits who are the land itself, but also relate to fertile soil, plants, abundance, and in this way a successful "fate". As embodiments of land, this sometimes correlates with traditions where the fey run back and forth on the waters edge of the sea coast − because they are the land but not the salt-water sea. But the fey do seem to be fresh water lakes and streams and so on. The Celtic concept is, they live above and below the surface simultaneously, as a mystical union of opposites. So below an earth mound that is above the rest of the land is a home of the fey, or a cave where above the land is also below it is also a home of the fey. Relatedly "pixie dust" is actually dirt, so that a person within its cloud is in the magical liminal fey space that is simultaneously above and below land. A person who wears a sod of grass-rooted soil on as a hat is simultaneously above and below the earth surface, thus can become magically invisible or fly. A boulder that is on top of the soil, becomes above and below simultaneously, and a fey home. Similarly, there is a tradition where below the water surface but above the riverbed or lake basin is also this liminal fey space.
Previously I interpreted drow as fey who are land spirits. Thus live in caves, simultaneously both below and above the land surface. Also they are the color of the fertile soil, often near black.
I definitely view the drow as having always been black, especially when good before the influence of Lolth. I am glad 5e does too. The race description says the drow descend from earlier dark-skinned elves. (This contradicts the Gygax tradition that said dark skin is a curse.) Indeed, the dark skin is a fey blessing embodying the lifeforce of fertile soil.
Where the aeven drow now live under the surface of a glacier but above the surface of the earth, it continues the Celtic fey concept. It is no accident that the presence of drow can make even the glacial arctic a fertile life-abounding location.
Where the loren drow are under a canopy of fertile plants, this too might count as both-above-and-below the surface − especially if these canopies carry soil upward for colonies of plants to live in the sunlight.