Ecologists are reviewing research on reefs around the Hunga Tonga-Hunga Ha'apai volcano, to see how biodiversity may fare following the latest eruption.
Underwater research at Hunga Tonga-Hunga Ha'apai. Photo: Supplied / Patrick Smallhorn-West
When it erupted from December 2014 to January 2015, water temperatures temporarily changed by up to 5C and masses of ash fell into the sea.
The crater created an 185 hectare landmass - the newest landmass on earth - connecting two islands.
Scientists went there four years later to see if surrounding coral and fish populations survived the explosion.
The research was part of reef ecologist Dr Patrick Smallhorn-West's PhD.
Seeing the latest eruption had been "surreal", but Smallhorn-West was optimistic coral populations could recover around the caldera, and said it was just a matter of how fast.
"If some have survived, then you'd probably get decent recovery happening pretty quickly. If the whole thing has been annihilated, then it might be a bit longer ... I hope to see actually pretty decent recovery around there, in the next five years or so if there are no further eruptions."
Patrick Smallhorn-West. Photo: Supplied / Patrick Smallhorn-West
He and fellow scientists found that after the last eruption "in some places the reefs were completely annihilated, kind of the worst thing that can happen to a reef, it's like a nuclear bomb".
But another section "had been completely undisturbed" with "reef growth there that was probably 40 or 50 years old".
"It was the healthiest we'd seen in the whole country."
He believed that was due to the reef's isolation - there is less pollution and fishing - and healthy, older parts of the reef have helped the damaged parts recover quickly.
"There was really high recruitment, lots and lots of coral babies growing back where it had been destroyed. To an extent that was quite surprising, actually. So now with this eruption, the satellite footage is showing some of the reefs that we dove on in 2018 look like they're actually out of the water now or just gone," Smallhorn-West said.
"So those ones probably don't have a good chance, but the outer edge of the northeastern island, that's where it had been totally protected from the earlier eruption. So it'll be interesting to see what happens there."
Smallhorn-West said the volcano was "an incredible part of the world".
Researchers walked around the volcano during their field work. Photo: Supplied / Patrick Smallhorn-West
"There's two pre-existing islands, and hence the name Hunga Tonga-Hunga Ha'apai and then in 2015, this whole thing came up in the middle and joined them together.
"So there were lots of plants, recruiting and there was whole colonies of seabirds starting to nest there in all the cracks. And it's kind of sad thinking a lot of that's probably not there anymore," he said.
"I think these islands, Tonga's volcanic archipelago, has a wonderful chance - or it should be considered for - a UNESCO World Heritage listing."
The 2018 research team spent days diving the reefs, and a few hours exploring around the crater.
"It was pretty cool, pretty stark, and some pretty massive cracks that you wouldn't want to fall into, and it's all kind of on consolidated ash and rubble."
Smallhorn-West said there were "all kinds of reasons to keep studying these places" and he hoped to do more research on the volcano's reefs.
But he said disaster recovery was the immediate priority and the primary concern was the safety of all those affected by the eruption.