That which digs down through accumulated ground to bring up what was buried. It moves far more than it keeps.
Latin excavare (to hollow out), from ex- (out) + cavare (to hollow), from cavus (hollow, a cave). The archaeological sense — digging to recover what is buried — is a 19th-century specialisation of the plain older meaning, which was simply to make a hole.
The instrument that recovers what the house has already built and half-forgotten. The archaeological reading is the operative one: excavation does not CREATE what it finds, it removes what has covered it — which is why the tool reports rather than invents.