The Magmoor Caverns are a mostly subterranean area of Tallon IV flooded wth magma. It is directly connected to every explorable area in the game except for the Impact Crater, and it is the only area connected to the Phendrana Drifts. It is also the only area without a boss battle.
As a whole, the Magmoor Caverns are long and narrow, and the extreme temperatures necessitate the use of specialized heat protection equipment. (Though the magma will hurt Samus on contact regardless of the suit she's wearing) The area contains a minor Space Pirate base.