I heard about McGahan's imminent demise a few months ago but somehow did not register the posthumous release of this novel. That goes to show that I was premature in ranking his output.
Briefly: presumably drawing inspiration from David Walsh, McGahan takes us on a ride of fanciful geography and history to the Southern Ocean — not too far from Hobart — where a billionaire mountain climber has built a "submerged" mansion in the summit of a 3km high mountain that stands about ten kilometres away from the 25km high "Wheel" that he conquered in his youth. Layered on top is a weak parapsychology which powers a revenge plot overstuffed with horror tropes. The writing is his usual: loads of foreshadowing, overly repetitious and occasionally quite fine.
The main flaw in this work is that it is a puree of so many other things. The mountain climber as Ozymandias. The billionaire as Bruce Wayne. Elements of a James Bond villain, mental instability where previously there were none, an anguished planetary consciousness like Asimov's Gaia before Daneel. Innuendo reported as interleaved excerpts, somewhat like a fat Brunner. Architectural fetishism; the main character is approximately the daughter of Frank Lloyd Wright. The neatly folded clothes of Jasper Fforde. The Sherpas. Tales from the Crypt moralism. There's an Alien: Prometheus vibe, and of course Solaris and doubtlessly other disaster movies I haven't seen. Drugs are bad ok. Portal!
A less severe flaw is that the characters all have the same voice — McGahan's — and it often feels like he's talking to them and not us for we learn little. Another is that the whole thing reads like the Risks Digest doesn't exist. Yet another is the sheer quantity of Chekhovian devices, many of which went off without motivation.
I'm going to rank it at 4.5 — above Underground and below The White Earth.
James Bradley reminded me that we've seen some of this before, in Wonders of a Godless World. Reviews are legion and typically fawning. Much later (2019-11-15), James Ley provides a broad retrospective.