Thanks, henkeeper, for the heads up on the position of the thematic hint. I'm so glad that I worked out how to enter down solutions myself early on as it greatly helped to solve many of the across clues. This "hint" seems anything but! Went to bed with only 17d to crack, having all the elements of the solution apart from four letters for 'mountain'. ETNA didn't fit so I trawled through a list in Wikipedia—did you know there's a Mount Massive in Colorado? That's like a physicist discovering a subatomic particle and naming it 'tinyon'!
I woke up with correct term for 'mountain' in my head...