Screen adaptation now. James Cameron actually went to the Challenger Deep for some insane rich-people reason, he should be eating this up. It's got monsters, it's got dread, it's got bitter not-quite-divorcées. There's even a submersible descent so claustrophobic I wanted to get off the tube early.

An armoured research vessel sails to the deepest part of the ocean to hunt mermaids. It's not a spoiler to say they find them! Grant is not relying on shadows and glimpses to keep her beasties formidable; they're feasting on-page before the Chapter 1 heading. These things have 4 knuckles, move like eels, and can chatter the final screams of your friends as a lure. In the same way Tolkien wrote a book for his language, it feels like Grant wrote Into the Drowning Deep to educate us about mermaids. Linguistics, behaviour, anatomy - all are detailed convincingly enough for this layman. It feels like David Attenborough should be narrating their migration in the next installment of Blue Planet.

An unfortunate consequence of how vivid the mermaids are is that the humans pale in comparison. They looked great at the starting lines - a mermaid truther/company man pair of estranged spouses, a trio of siblings arguing in ASL over whether one should try to make the Challenger Deep descent, a devoted husband-wife duo of ruthless big game hunters - but their promise never crystalised into the drama I was salivating for. Tory, the sister of an mermaid victim whom we follow closest, never felt like much more than that. (Her romantic interest, a cosplayer-turned-media-personality had more to her though, and the Sapphic representation is always nice.)

All the secondary characters and extras have an acute case of Prometheus Syndrome. If had just discovered I was floating over a nest of apex predators with a record of eating people, I would not ignore weird sightings in the water, ignore lockdown procedures, or really even step foot on deck. 
 
The writing was evocative and vivid when it needed to be (when the mermaids were going to town), and never missed a chance to say something ominous about the sea. It could have missed a few of those chances and been tighter for it, in my opinion. Shave off 20% of the length. Maybe do more with the human relationships and

captive mermaid

instead of telling us yet again how the sea would never forgive us for leaving it. Chilling stuff, but anything can get repetitive when its consistently taking paragraphs out of the narrative.

Small complaints, all in all. If you've ever shuddered when something touched your foot underwater, this book'll be a ride.