This one started with the clue/answer pair at 18 Across18AWhen words collide?[-----------]. 23 Down23DIs a catalytic converter, maybe?[-----------] went in shortly thereafter, and it’s one of my favorite clue/answer pairs in any of my puzzles, if you'll excuse the self-pat on the back.
51 Down51DAltamira or Lascaux, e.g.[----]’s clue (“Altamira or Lascaux, e.g.”) references something on my bucket list that I’ll never be able to fulfill. I’m fascinated by physical anthropology, and the art in these locations could illustrate the dictionary definition of “evocative.” Alas, they’re closed to the public, so I’ll have to content myself with listening to the second track on The Royal Scam (link here: youtube.com/watch?v=m_5MtGCWImE&ab_channel=SteelyDan-Topic ), where Steely Dan captures some of the magic.