[andrewinardeer]

Surely plate tectonics after a few billion years would move these disc brakes down into the Earth's mantle and melt them?

[nemo]

We have fossils that are > 3 billion years old. While subduction does steadily erase the past, not everything subducts. See: https://en.wikipedia.org/wiki/Craton

[thfuran]

That depends where you leave them.