地幔不是液体，它是一种触变物质，当压力降低时流动，流体可以流动。如果有大量的高熔点的矿藏，可能会有口袋。但是洞穴需要一个“地板”。

The mantle is a convecting solid which can deform over long timescales. Let's assume that such a cavern did somehow form. Whatever it is filled it, would be of lower density than the surrounding rock. It would slowly rise upwards through the solid-yet-deformable mantle until it reaches a place where the rocks are brittle, not ductile. That place is the crust. And as you know, the crust is full of caverns and there is no problem with that.