|This article or section contains minor spoilers. You may want to avoid reading it.|
The Magma sea is a large, multi-level 'sea' of magma that expands across the entire world. These can typically be found by around Z level -120 (though this seems to vary wildly (minimum -5 maximum -450 z level)Verify. The magma sea is always found below the last Cavern level.
It is not uncommon to find multiple z-levels of pillars of Raw adamantine in the magma sea (in fact, a 3x3 embark square guarantees at least one).
Any items or natural rock walls dropped into the magma sea (specifically, onto "magma flow" tiles above semi-molten rock) will vanish without a trace.