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White Island is a popular tourist destination
Moments before one of New Zealand’s most active volcanoes erupted on Monday, tourists were seen walking inside its rim.
The privately-owned White Island, or Whakaari, is a popular destination for day tours and scenic flights. It has been dubbed by some tour operators as a “living, breathing, geological giant” and “the world’s most accessible active marine volcano”.
Five deaths have been confirmed since Monday’s eruption. Twenty-three people were rescued but others are reported missing and police have said there is “no sign of life” on the island.
The incident has raised questions over the safety of the White Island tourism industry and the reliability of volcanic warning systems.
Here, two experts explain the background of the volcano and its latest eruption.
Where is the volcano and how active is it?
The volcano is located in the Bay of Plenty, about 48km (29 miles) from the east coast of New Zealand’s North Island.
Jan Lindsay, a professor at the University of Auckland, said White Island was “persistently active in the sense that it has a very active hydrothermal system”.
Rather than having lots of eruptions involving magma, she said this meant it saw periodic ash explosions and had lakes “churning over with gases”.
“The spectacular thing about White Island is that there is so much gas coming out of the volcano and lots of minerals crystallising,” said Ben Kennedy, a professor of physical volcanology at the University of Canterbury.
“What that also means is that there are minerals crystallising deeper down and creating blockages – that allows pressure to build, and the volcano needs to periodically clear its throat.”
He said there had been six small eruptions in the past eight years, but that on previous occasions there had been no tourists on the island “mostly due to the time or weather conditions”.
Ms Lindsay noted that the ruins of a house could be seen behind a group of people trying to escape from Monday’s eruption in a video shared on social media.
“That was destroyed in 1912 in a previous eruption. There used to be a sulphur mine. Between 10 and 20 people died,” she said.
Was there any warning?
Experts said there was a warning of unusual activity at the volcano ahead of Monday’s eruption.
In the weeks leading up to it, the alert level was raised from one – meaning minor volcanic unrest – to two, indicating moderate to heightened volcanic unrest.
“There was a heightened level of unrest and everyone was aware,” Ms Lindsay said. But, she added, “even though there was increased activity, there was no sense of what was going to happen.”
She said it was possible that there was no magma involved in Monday’s eruption, which may have made it harder to detect.
“If you have something that’s being driven by the hydrothermal system it’s… not like when you have a magma chamber building beneath the volcano and you get lots of seismic activity,” she said. “If it’s a shallow burp, you may not see that.”
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Mr Kennedy said the volcano had been in a “state of heightened activity” for the past eight years. “The type of volcano it is means small eruptions can happen at any time with any warning,” he noted.
But, he said, high levels of sulphur dioxide were the “tell-tale sign” that prompted the alert level to be upgraded.
How big was the eruption?
Monday’s incident has been classified as a level three “minor volcanic eruption”.
Mr Kennedy said this was because it only affected the crater area.
About 70% of the volcano is under water, with the crater and surrounding peak standing some 321m (1,053ft) above sea level.
Including its underwater base, White Island is the largest volcanic structure in New Zealand.
How is tourism to the island organised?
Officials said about 50 visitors were near the volcano when it erupted on Monday.
The island was bought by Auckland stockbroker George Raymond Buttle in 1936. He later refused to sell it to the government but agreed in 1952 to have it declared a private scenic reserve. The island is today still owned by the Buttle Family Trust.
According to the New Zealand Tourism Guide, the owners of White Island Tours were appointed as the official guardians of the island and access is only granted through designated tour operators.
The BBC has contacted White Island Tours for comment.
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Government agency GNS Science “put out their alert bulletins and have good communication with tour companies and they know what the risk is,” Ms Lindsay said.
“They go out so often and are used to the volcano’s behaviour, but something like this – it happens and it has happened in the past, but you can go years without it happening. It may change the way they do things in the future.”
How are people killed by volcanoes?
People nearby a phreatic (steam) eruption are typically killed by “hot steam and water and what we call ballistic projectiles – blocks being thrown out of the vent”, Ms Lindsay told the BBC.
Mr Kennedy compared the incident to the 2014 eruption of Japan’s Mount Ontake, where most people hurt were hit by flying rocks ejected out of the volcano.
“There also tends to be a cloud of ash and gas and rocks that might be moving sideways out of the crater,” he said.
What is it like on the island?
White Island reportedly got its English-language name from explorer Captain James Cook in 1769 because it always appeared to be in a white cloud.
A number of stories have been written about the volcano and in more recent years it provided a backdrop to movies including the third film in The Chronicles of Narnia series.
“It’s a spectacular otherworldly place,” said Mr Kennedy.
“It’s an environment that you should wear gas masks on because of strong sulphuric gases. You can hear the noise of the gas coming out of the volcano. It’s quite an intense but beautiful experience if you do a little circuit and then get off pretty quickly.”