Romy Ash’s debut novel, Floundering, has sat on my bookshelf since the Sydney Morning Herald, where I worked as literary editor, named her as one of the best young Australian novelists in 2013 – the year she was shortlisted for the Miles Franklin literary award among others.

The Australian author Cate Kennedy wrote of the neglected young brothers in Floundering: “These boys are so real you will lie awake worrying about them” – words so true that I still feel anxious for them.

Like everyone who saw Ash’s talent, I’ve been waiting for the next novel while she did academic work, published essays and a children’s book, raised her own children, lost loved ones and watched Earth’s health decline.

All that living has gone into Mantle, a more complex novel of ideas that shares DNA with Floundering, in an exquisitely wild, watery depiction of humans and nature under pressure.

Mantle is set on the coast of south-eastern Tasmania, where 50-year-old Ursula comes to see her dying mother at her clifftop house overlooking the D’Entrecasteaux Channel. Delores retreated to a simple hippy life after the traumatic death of her other daughter and husband, but turmeric and bone broth can’t save her from a mysterious growth in her lungs.

Ursula has an academic paper to write for her work as a stratigrapher: a geologist who reads the planet’s history in layers of rock and the fossils they contain. Now, she notes grimly, we are leaving “the plastics layer”.

But there are many distractions. By the time Delores dies breathlessly, Ursula has a bumpy rash moving across her skin and so do others in the small communities along the shore. In a replay of the Covid-19 pandemic, state borders close, supermarkets empty, and Ursula is isolated at her mother’s house with a pantry well stocked since the millennium-bug panic.

A more pleasurable distraction is sex with Toby, a young man who dives for dead fish at the salmon farm, which pumps out toxic water and engine noise. Ursula is attracted and appalled, taking comfort in his no-strings company. But it’s not so simple: they wake up stuck together by tendrils growing from their skin, and have to tear painfully apart.

Mantle becomes increasingly strange as the rash erupts into clumps of mushrooms, alarming yet beautiful. Ursula learns that fungi are proliferating in the warming atmosphere, their spores are in the air we breathe and their mycelium rooted in our bodies. They are harbingers of decay and, perhaps, a post-Anthropocene epoch.

Scientific language and surreal images merge in an uneasy but not implausible scenario, filtered through Ursula’s inquiring mind. Ash writes into the crowded genres of climate-change dystopia and Tasmanian gothic, using the island state’s remoteness and its dark history of colonisation and extinction. How will she stand out?

As in Floundering, she excels at sketching a small cast of believably quirky characters. The lonely old drunk Ernie, for example, is a fisherman trying to regenerate the kelp forests that once fed lobsters.

Most striking are Ash’s sensual sentences about damp landscapes and warm domesticity. Ursula recalls “quolls splattered across the road in their spotty pyjamas” and sees rocks “laminated, like croissant pastry”. “I go inside to the kitchen, lift my hoodie and cut the mushrooms off with a knife. Thick stipes, darker gills, brown blush on the top.”

Ursula is obsessed with food, endlessly hungry as she feeds her fungi. She uses her culinary skills to roast a chicken, brew coffee, prepare mussels and omelettes, all observed in mouthwatering detail. Cooking is an act of survival and love.

Equally evocative are scenes in which Ursula overcomes her aversion to swimming. Until now her knowledge of sea creatures has been gleaned through fossils. In deep water she absorbs the vibrancy of kelp, coral and sea dragons. “Little silver fish swim around my hands; they’re tiny, scraps of sparkle.”

Ash works powerfully, if not subtly, at the level of metaphor. The title of Floundering referred to the act of fishing for flounder and to a mother out of her depth in caring for her vulnerable sons. Mantle – as Ursula spells out – is both a protective covering and the invisible layer of the Earth between crust and core.

Ursula’s grief reaches into long-buried memories and intensifies her responses to environmental distress. Perimenopause gives her a late flare of lust and courage. Flashes of wonder break through the gloom, keeping up a rhythm of tension and relief. Ash divides the novel into sections titled Civil, Nautical and Astronomical, after the phases of twilight that precede complete darkness. The short final section, Night, is a tour de force.

Disturbing as science and exhilarating as art, Mantle creates a bizarre, brilliant vision of the near future. The image of Ursula and her mushrooms will make my skin tingle for many humid Sydney summers to come.