Australia's meteorological service said a "major eruption" took place at the Hunga Tonga-Hunga Ha'apai volcano on Monday, but no tsunami warnings have been issued.
Saturday's eruption was probably the largest recorded anywhere on the planet in more than 30 years, according to experts. Dramatic images from space captured the eruption in real time as a huge cloud of ash, gas and steam was spit up to 20 kilometers (12.4 miles) into the atmosphere - and tsunami waves were sent down over the Pacific Ocean
Mass casualties have not yet been reported, but aid organizations are concerned about polluted air and access to clean water for people on the remote islands of Tonga.
With communications down, Australia and New Zealand sent planes to investigate the damage.
Here's what we know about the eruption and the fallout.
Where is Tonga's Hunga-Tonga-Hunga-Ha'apai Volcano?
Tonga is a Polynesian country with more than 170 islands in the South Pacific and home to about 100,000 people. It is a remote archipelago located about 800 kilometers (500 miles) east of Fiji and 2,380 kilometers (1,500 miles) from New Zealand.
The Hunga-Tonga-Hunga-Ha'apai Volcano, about 30 kilometers (20 miles) southeast of Tonga's Fonuafo'ou Island, lies underwater between two small islands about 2,000 meters (6,500 feet) high from the ocean floor, with about 100 meters (328 feet) visible above sea level.
Researchers said it has erupted regularly over the past few decades.
The most recent eruption began in December 2021, when gas, steam and ash flags rose about 12 kilometers (7.5 miles) into the air. The volcano erupted again on January 14 and a massive eruption on January 15 sent shock waves around the world and triggered tsunami waves across the Pacific Ocean.
Where did the tsunami hit?
The eruption caused a tsunami on Tonga's largest island, Tongatapu, with waves recorded at 1.2 meters (about 4 feet) near Nuku'alofa town, floating out on coastal roads and flooding properties on Saturday.
Tsunami warnings went into effect across Pacific island nations, including Fiji, Samoa and Vanuatu. Footage from the ground in Fiji shows people fleeing to higher ground in the capital Suva while big waves hit the coast.
Tsunami warnings and warnings were also issued from parts of New Zealand, Japan and Peru, to the United States and Canada's British Columbia.
In Japan, northeastern Iwate prefecture experienced waves as high as 2.7 meters (9 feet), and several minor tsunamis were reported in several other places, according to public television station NHK. By Sunday afternoon, all tsunami warnings had been lifted in Japan.
The eruption also sent waves to the U.S. west coast, with some over 3 and 4 feet in height, according to the National Weather Service office in San Diego. Tsunami waves could be felt in California, Alaska and Hawaii.
What happens to the ash cloud?
A giant volcanic ash cloud covered Tonga over the weekend, darkening the afternoon sky and covering Nuku'alofa in a thick foam of volcanic dust on Saturday.
Save the Children said drinking water supplies could be contaminated by the ash and smoke, and the immediate concern in Tonga is air and water safety.
The ash cloud drifted west and was visible over Fiji, Vanuatu and New Caledonia on Sunday. By Monday, it had reached Australia's Queensland, according to the State Meteorological Service.
"If you noticed a particularly amazing sunrise, it was the sunlight that was scattered by #volcanaske from the eruption in #Tonga," the Queenslands Bureau of Meteorology said on Twitter.
The ash prevented an Australian reconnaissance aircraft from departing to assess the damage in the early hours of January 17, although the aircraft took off later that morning.
Several flights from Australia, New Zealand and Fiji to Tonga were postponed due to the ash cloud.
Early data suggest the volcanic eruption was the largest since the explosion at Mount Pinatubo in the Philippines in 1991, New Zealand-based volcanologist Shane Cronin told Radio New Zealand.
"This is an eruption that can best be seen from space," Cronin told Reuters.
"The large and explosive lateral spread of the eruption suggests that it was probably the largest since around the Pinatubo eruption in 1991," Cronin said.
What is the extent of destruction?
There have been no reports of mass casualties in Tonga so far, and the extent of the damage is still unknown, as communications - especially in the remote islands - have not yet been re-established.
Tonga "needs immediate help to provide its citizens with fresh drinking water and food," the country's President of the House of Representatives, Lord Fakafanua, said in a statement posted on social media.
He said "many areas" had been affected by "significant volcanic ash falls", but "the full extent of the damage to life and property is currently unknown."
New Zealand's Prime Minister Jacinda Ardern said on January 16 that tsunami waves had a "significant impact" on Nuku'alofa, with boats and large boulders washed ashore. "Shops along the coast have been damaged and a significant cleanup will be needed," she said.
The main subsea communication cable has also been affected, probably due to power loss.
Australia's Minister for International Development and the Pacific, Zed Seselja, said there was "significant property damage" in Tonga, including on roads and houses. He said there is still "very limited, if any" information coming from the outer islands.
New Zealand's High Commission in Nuku'alofa reiterated this, saying in a statement on Monday that "significant" damage has been reported along the west coast of Tongatapu.
A family-owned beach resort located on Tongatapu also said the business had been destroyed. "It is with sorrow to say that our beautiful home Ha'atafu beach resort has been completely wiped out. The entire western coastline has been completely destroyed along with the village of Kanukupolu," Ha'atafu Beach Resort wrote on Facebook on Monday.
The International Federation of Red Cross and Red Crescent Societies said its team is on the ground and has enough supplies in the country to support 1,200 households.
"From the small updates we have, the scale of the devastation could be huge, especially for remote islands," said Katie Greenwood, IFRC's Pacific Head of Delegation, according to Reuters.
New Zealand's defense force sent an Orion aircraft to Tonga on a surveillance mission to assess the damage. Ardern said the country has committed an initial $ 340,000 in emergency supplies, technical support and supportive local responses.
Australia said it was preparing for further support, with an aircraft loaded with humanitarian supplies such as water and sanitation kits ready to deliver to Tonga when conditions allow.
China and the autonomous island of Taiwan said in separate statements that they are willing to provide assistance at Tonga's request.
CNN's Rhea Mogul, Alex Stambaugh, Hira Humayun, Eric Cheung, Jake Kwon, Aliza Kassim, Teele Rebane and Akanksha Sharma contributed reporting. Additional reporting from Reuters..