Curiosity drills 6.4 cm (2.5 inch) holes on Mars. Comet harpoons have been proposed for future space probes. There are a number of ways in which we can penetrate the surface of terrestrial bodies, but I'm interested in what is the deepest we've gotten so far. As of April 2018, how deep have we gone (measured in vertical distance below surface)?
(To clarify, by "terrestrial" I mean to include all dwarf planets, comets, asteroids, moons, and terrestrial planets, unless by some marvelous exception it would stretch the definition too far--like a gas dwarf planet, were one to exist.)