(CNN) When the Hunga Tonga-Hunga Ha'apai volcano erupted in Tonga in January 2022, it became the largest eruption ever recorded with modern technology.
The blast, estimated to be hundreds of times stronger than the Hiroshima nuclear explosion, was heard in Alaska, more than 10,000 kilometers (6,000 miles) away. A plume of ash, smoke and volcanic matter shot 58 kilometers (36 miles) into the air, and hurricane-speed winds were reported in Earth's uppermost atmospheric layer at the edge of space.
And then came the waves: tsunami warnings were issued in nearby Pacific Island nations Fiji, Samoa and Vanuatu, as well as farther away in New Zealand, Japan, Peru, the US and Canada. The ensuing tsunami devastated Tonga with waves up to 15 meters high, killing three people and causing an estimated $90.4 million worth of damage.
Now, a team of researchers has completed the fullest investigation to date into the event, confirming that almost 10 square kilometers of seafloor was displaced -- equivalent to 2.6 million Olympic pools, and a third more than initial estimates.
However, they found that only three quarters of this material was deposited in an area within 20 kilometers (about 12 miles) of the volcano, leaving a sizable chunk unaccounted for. New Zealand's National Institute of Water and Atmospheric Research (NIWA), which conducted the research, believes this missing debris could be partly explained by "aerial loss," which is why it wasn't noticed until detailed mapping work was completed. The material was shot into the sky and remained in the atmosphere, circulating for months, explaining why it wasn't on the seafloor.
But it remains unclear to researchers exactly why the eruption was so explosive.
Some answers came from an earlier expedition also conducted by NIWA, which mapped the sea floor around Hunga Tonga-Hunga Ha'apai.
Erica Spain is a marine geology technician at NIWA and a member of the expedition that set out in April. Describing herself as a "seafloor detective," Spain uses high-tech echolocation machinery to hunt for underwater volcanoes, and pieces together clues about the "triggers" that influence underwater eruptions.
On board the RV Tangaroa, NIWA's research vessel equipped with state-of-the-art underwater surveillance technology, Spain had a dual role: operating the multibeam and taking samples of mud and rock sediment from the seafloor.
The multibeam echosounder "sends out acoustic pulses to map the seafloor in 3D," says Spain, likening it to the echolocation a dolphin uses. "We have hydrophones that receive that echo, and from that, we can determine how deep the sea floor is, and build up an idea of its shape and geometry."
With a volcanic cone just 100 meters (328 feet) tall on a small island in the Pacific, Hunga Tonga-Hunga Ha'apai wasn't much to look at before the eruption. Beneath the surface though, the volcano stretched over 20 kilometers wide and nearly 2 kilometers deep.
Continued instability in the caldera meant the crew couldn't examine the volcano's opening — so an uncrewed surface vessel was deployed instead. Operated remotely by a SeaKit, based in the UK 16,000 kilometers (10,000 miles) from Tonga, the 12-meter-long (40-foot) robot discovered that the caldera had collapsed, now lying 700 meters (2,300 feet) below the surface.
In total, 22,000 square kilometers of the seafloor were scanned. Parts of the seafloor around Hunga Tonga-Hunga Ha'apai had already been mapped out, and by comparing the maps of the seafloor before and after the eruption, "we can begin to build up a better image of what these triggers might be," Spain says.
The results of the expedition surprised the team, says Spain. They had expected that the huge eruption would have left a lot of volcanic debris on the seafloor, but in fact, "the volcano looked very similar to what it looked like decades prior," says Spain.
Instead of coming to rest in the ocean, "a large proportion of that (volcanic material) just went straight up into the stratosphere," says Spain.
The discrepancy between the size of the caldera collapse and the seafloor debris pointed toward another factor in the explosion: hot magma from the volcano had interacted with the cool seawater to create steam. "Steam takes up a thousand times as much volume as water," says Spain. "It explains in some regard why it was so eruptive."
The results from the data gathered before, during, and after the eruption pointed towards "multiple mechanisms" happening simultaneously, says Emily Lane, a hydrodynamic scientist at NIWA and member of New Zealand's Tsunami Experts Panel.
The initial eruption blew water out of the way, causing localized waves. But changes in air pressure reinforced those waves creating a meteotsunami that travels faster than the speed of sound — and in the case of Hunga Tonga-Hunga Ha'apai, "it generated a pressure wave that traveled around the world three or four times," says Lane.
Waves were also created by volcanic debris raining down into the water, and the caldera collapse, says Lane.
NIWA already has a tsunami warning system in place, with sensors on the seafloor around New Zealand and the South Pacific to monitor sea levels, tides and currents, and to report anomalies. And now, the data gathered from Tonga can help refine these sensors, says Lane.
"This event has really changed our understanding of volcanic tsunamis because this is the first time that we've actually been able to get modern instrumentational measurements of what happened," says Lane.
And while the map of the seafloor gave researchers a better understanding about the eruption at Hunga Tonga-Hunga Ha'apai, it also contributed to a larger project: Seabed 2030, a global initiative run by the Nippon Foundation, which aims to map the entire sea floor.
This map of the seafloor can help identify important or vulnerable ecosystems, says Spain. The information gathered can also help aid the recovery of the surrounding ocean and marine environment. For somewhere like Tonga, where around 82% of the population engages in subsistence fishing, understanding the impact of eruptions on aquatic life is vital.
"We don't know enough about the ocean and our impacts on it, so mapping it, observing it, and understanding it is incredibly important," says Spain.