Elemental Chaos
The Elemental Chaos is part of the plane cosmology of Dungeons & Dragons 4th Edition, replacing the Elemental Planes introduced in the 1st edition and given their truest time to shine in Planescape.
Like a lot of 4e's controversial shit, the Elemental Chaos was born out of WoTC's decision to take an objective look at sacred cows of editions past and ruthlessly slaughter them if they deemed it necessary. For the longest time, the Elemental Planes of D&D had been pretty much ignored in favor of the Outer Planes; even Planescape tended to focus predominantly on the Outer Planes and treat the Inner Planes as a source of monsters. When they began, they were literally nothing but infinite, empty expanses of pure earth, air, water or fire; Planescape made them at least interesting to adventure in by adding some traces of the other elements (the City of Brass, for example, or floating islands in the Plane of Air).
Therefore, WoTC came to the decision that the old Inner Planes really served little purpose other than grid-filling and monster-generators. So, as part of the World Axis cosmology, they smashed them together, added a dash of Limbo, and created the Elemental Chaos: an ever-shifting plane of elemental substance, where reality can change at any moment and elemental matter isn't bound by the forces of mundane reality. Trees of fire and lighting, rivers of liquid metal, mountains that grow upside down, all the most fantastic vistas imaginable literally smashed together and capable of changing into just about everything.
The Elemental Chaos is one of the most important parts of the setting; birthplace of the Archomentals (known in-universe as Primordials), it is literally the foundation of the material plane, as all the "middle tiers" of the World Axis cosmology were built from the materials of the Elemental Chaos. It is the home of the Genasi and the Githzerai, and a source of many dangers.
Of course, with the controversy that 4e elicited, the Elemental Chaos underwent a huge makeover in 5e; the Elemental Planes were brought back, though styled more after the Elemental Poles of Exalted, with the Elemental Chaos being the "foundational border", the region beyond the Elemental Planes where all elemental matter originates and wildly mixes, separating them from the Outer Planes beyond.