harry wrote:

When fossil fuel are sufficiently depleted, you will be living in a
cave. Unless we have a substitute.

The process may already have begun. I hear a lot of people in America
don't even have a cave and must live in tents.

Oh, bother.

Rome denuded North Africa for wood to make charcoal. The wood ran out.
Later, Europeans did the same to Europe. The forests disappeared. But a
substitute was found.

Coal powered the Industrial Revolution and coal is still a big player. But
oil was found to be more convenient and cheaper.

Nuclear power is a possible long-term substitute, but it's still in its
infancy.

IF - and that's a BIG "if" - we come close to running out of petroleum,
something else will come along.