Some interesting recent developments:
When a continental plate is subducted beneath another, you’d imagine that it would melt away to liquid hot magma and become part of the gooey inner mess. but as it turns out, some of these plates can stick around in a more or less solid state for a *long* time, and they can be mapped using seismic tomography. Around a hundred of these slabs have been mapped out, down to several hundred kilometers in depth, still recognizably more-or-less solid chunks drifting in the mantle. This means that there *could* *maybe* *possibly* be some really interesting things down there where we’ll almost certainly never get to see them. Say, 300 million years ago something truly remarkable evolved on some small continent… say, an archosaur developed smarts *real* fast and became human-smart and built a whole civilization on their island, then they were wiped out by disease or aliens or a high tax rate married to an unwise nanny-state system of government. Their cities of concrete and stone were buried under dust and mud and ash, then fifty million years later the plate was subducted. Meaning that a hundred miles below your feet there might still be recognizable evidence of a far-pre-human terrestrial civilization, one you’ll almost certainly never get to hear about.
These aren’t piddly little tubes, either. These are big enough to plant whole cities within, inside where they’d be protected from radiation and thermal cycling.