Over the last several months, I was continually seeing headlines promising a "deep dive" into some topic. (Search "deep dive" in Google News to see what random topics journalists and bloggers are diving into.) After seeing a phrase like that for the umpteenth time, the crossword part of my brain lit up and tried to make a puzzle out of it. Luckily, I was able to get a symmetric set of themers (full disclosure: I did not know "Challenger Deep" before looking for theme entries, but I appreciated learning about it) and I was able to get the mirror symmetry needed for this puzzle to work out. My thanks to David for his careful and creative editing, and I hope solvers enjoyed jumping into this puzzle.