Debénath & Dibble on the sheer scale of what lies beneath:

Imagine … that during the time of the Acheulian in Europe, which lasted for at least 500,000 years, there was a constant population of 5,000 active tool makers. Imagine also that each of these flintknappers made only ten bifaces per year and perhaps 100 flake tools. Even with such conservative parameters…, this would have resulted in the production of 25,000,000,000 bifaces and 250,000,000,000 flake tools, of which only a minuscule proportion has been collected during the history of Paleolithic research.

– André Debénath and Harold L. Dibble in Handbook of Paleolithic Typology: Lower and Middle Paleolithic of Europe (1994)