Limbo

Limbo is the outer plane of chaotic neutral: a seething mix of air, water, earth and fire in constant flux, which holds a shape only while a strong mind imposes one. It is home to the slaadi, and the githzerai build their fortress-monasteries there by sheer discipline. It sits at the right-hand end of the line that divides the upper planes from the lower.

Reaching it. By a jet colour pool in The Astral Sea, a swirling black curtain in the Deep Ethereal or a plane shift fork of nickel tuned to C.