Drilling into Earth’s mantle

There’s a news story about a new project which will attempt to drill into the Earth’s mantle. If it succeeds it will be the first time humans have done so.

This is surprising to me. Humans have walked on the moon. They have landed roving robotic explorers on Mars, and landed a probe on Saturn’s moon Titan.

But then below us is our vast Earth and it seems like we’ve barely scratched the surface. Why?

It seems like there should be some kind of Apollo project to drill way down into the Earth and learn more about our planet. I wonder how far it would actually be possible, with human technology, to drill down into Earth’s interior?

I also wonder whether it would be possible/realistic to build any kinds of structures which could withstand the enormous stresses that must occur at deep levels. My guess would be no, that the pressures and forces are simply way too enormous for any sort of lasting structures – such as elevators or conveyor systems – to be able to be built. Or?

Wouldn’t it be cool though to be able to create large, deep, cavernous holes that go way down – perhaps near the equator where there would be more light available to shine deep down into them – and then perhaps build multi-level cities inside them? Kind of like Pau City. Maybe somewhere in the Sahara desert would be good, if it would be possible to tap into some really deep water table in the Earth.