I'm surprised you even know what that word means. [And with those words, the Master joins them, shining a light down the hole]

Anyway, we've a myriad of options-- most of them wrong. Galvanic corrosion, pitting, microbial, high temperature... Biogenic sulfide corrosion, while positively fascinating, is probably not the source. Pity.

Given the pattern and the smell [which, of course, only he can smell], I'd say a strong acid is the most likely culprit. [He shrugs, then grins] I'd hate to see what it'd do to somebody's face.