[5][6] The volcano rises around 2,000 m from the seafloor and has a caldera which – on the eve of the 2022 eruption – was roughly 150 m below sea level and 4 km at its widest extent.

Hunga Tonga–Hunga Haʻapai likely had a previous major explosive eruption in the late 11th or early 12th century (possibly in 1108).

[3][10] The volcano's base at the seafloor is approximately 20 km in diameter,[11] rising roughly 2,000 m towards the sea surface.

The caldera itself has eroded rapidly in the southeast, originally allowing an opening that flooded the crater with seawater to form a bay.

[23][24][25] On 16 March 2009, a submarine eruption near Hunga Tonga–Hunga Haʻapai began spewing steam, smoke, pumice, and ash thousands of feet into the sky.

[26][a] By 21 March, Tonga's chief geologist, Kelepi Mafi, reported lava and ash issuing from two vents – one on the uninhabited island Hunga Haʻapai and another about 100 m (330 ft) offshore.

The eruption had filled the gap between the two vents, creating new land surface that measured hundreds of square metres.

[30][31] The eruption devastated Hunga Haʻapai, covering it in black ash and stripping it of vegetation and fauna.

Local fishermen reported a tall white steam plume rising from the ocean over the undersea volcanic mount.

[33] The eruption continued into 2015, with a tall ash cloud rising 3 km (9,800 ft) into the sky on 6 January 2015.

[34] The eruption entered a new stage on 11 January 2015, when the volcano began sending ash plumes as high as 9 km (30,000 ft) into the sky.

Tongan officials established a zone 20 km (12 mi) in diameter around the island to protect visitors from rock, ash, and acid rain.

[40] Tongan officials declared the eruption at an end on 26 January,[41] after observing no new gas, ash, or rock emerging from the island vent.

Stuart went to Hunga Tonga with the help of the Spanish explorer Álvaro Cerezo, who provides castaway experiences to remote desert islands around the planet.

[44] In June 2017, French explorers Cécile Sabau and Damien Grouille landed the island from their sailing boat COLIBRI.

[45] This material was studied by scientists at NASA's Goddard Space Flight Center, led by Dr James B. Garvin.

[46] Another analysis of the samples showed that the volcanic ash that forms much of Hunga Tonga–Hunga Haʻapai reacted with the warm oceanic water around it.

This chemical reaction turned the ash into much harder rock, and volcanologists believed the island would last for several decades rather than be eroded.

[46] In October 2018, scientists visited the island and discovered that its surface was covered with gravel, sticky mud, and vegetation.

Four deaths were confirmed in Tonga, including a British woman whose body was found after she went missing when the tsunami struck.

[72] A survey of the caldera by a remote operated vehicle in August 2022 found continuing signs of volcanic activity.

[73] In August 2022, a NASA report on the January 2022 eruption of Hunga Tonga–Hunga Haʻapai stated, "The huge amount of water vapor hurled into the atmosphere, as detected by NASA’s Microwave Limb Sounder, The excess water vapor injected by the Tonga volcano... could remain in the stratosphere for several years... may have a small, temporary warming effect... would not be enough to noticeably exacerbate climate change effects.

"[74] Later it was found that the excess water vapor from the 2022 eruption would remain in the stratosphere for about 8 years, and help making the 2023 ozone hole one of the largest and most persistent in history.