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The magma sea is a large, multi-level "sea" of magma that spans the entire world. These can typically be found by around Z level -120 (though this seems to vary widely (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 3×3 embark square guarantees at least one).
In addition, around and under the magma sea is semi-molten rock, which is impassable. There are a few exceptions, however.
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.
Stockpiles may not be placed on a magma flow tile. However, the magma flow tile can be "destroyed" by creating a construction on top of it and removing the construction.