Guano

The term guano refers to the feces from birds and bats.

There are isolated islands, in the middle of Earth's ocean, that are the homes for vast colonies of birds, which, over millenia, left those islands covered in deep deposits of guano.

This guano was such a valuable fertilizer that it was strip mined, and shipped around the world. The richest deposits of bird guano had been mined by the late 19th century.

Bats left rich deposits of guano is some caves.