What is the Elemental Chaos like (visually) in D&D 5e?
The DMG more or less covers the air, water, earth, and fire planes, but says nothing on the Elemental Chaos nor what it's like.
My only guess is that it's something like Limbo, both being pure-ish chaos (Limbo has some patches of unchangeable bits).
If 5e hasn't covered this topic at all, I'm open to lore from the most recent edition on this topic.
Looking in the Dungeon Master's Guide for D&D 5E, the Elemental Chaos is described as "an unending tumult of clashing energies and colliding substance", where "pure elements dissolve and bleed together" (p. 52-53; p. 52). It is located on the "outer edge" of the Inner Planes, where the pure elemental planes dissolve into the extra-planar void.
However, this is not its first appearance. The Elemental Chaos is described as a fundamental plane (in which a collection of other planes are contained) in D&D 4E. In the Dungeon Master's Guide for that edition, it is described as being "full of mountains floating through the air, stone slabs drifting on rivers of liquid fire, and clouds of pure lightning" (p. 107). According to this edition, the plane could also house drifting monasteries, as it possessed some stability. A paragraph on page 160 gives a more broad perspective:
The Elemental Chaos approximates a level plane on which travelers can move, but the landscape is broken up by rivers of lightning, seas of fire, floating earthbergs, ice mountains, and other formations of raw elemental forces. However, it is possible to make one’s way slowly down into lower layers of the Elemental Chaos. At its bottom, it turns into a swirling maelstrom that grows darker and deadlier as it descends.
The disparity between the descriptions is of course due to the two editions considering two different cosmological models to be standard; the Great Wheel in 5E, and the World Axis in 4E (both being tied to the Forgotten Realms setting).
I hope this at least answers some of your questions. For my own part, I'd speculate that some of the attributes and narrative design functions of the Elemental Chaos have been taken over by the plane of Limbo in 5E; however, I think an important distinction is that, while the description for Limbo in 5E states that creatures with strong mental discipline (such as the githzerai) can maintain pockets of stability there, the Elemental Chaos in 5E has no such attribute.
