Image copyright AFP PHOTO/MICHAEL SCHADE Image caption White Island is a popular tourist destination

Moments before New Zealand's most active volcano 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 a "living, breathing, geological giant" and "the world's most accessible active marine volcano".

Six people have been confirmed dead since Monday's eruption and eight are still missing on the island - though police say they are also likely to have died.

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.

GeoNet, New Zealand's geological hazard information site, says White Island has been classified as New Zealand's most active volcano since 1976, when it began 24 years of almost continuous eruption. A second "eruptive episode" started in 2011 and continues today.

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".

In 1914, a landslide destroyed a mining village on the island and killed 10 workers.

Was there any warning?

There was a warning of unusual activity at the volcano ahead of Monday's eruption.

In November, 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," Prof Lindsay said. But, she added, "even though there was increased activity, there was no sense of what was going to happen."

An eruption can occur at any volcanic alert level, but there are also times when a volcano alert is raised to level two and there is no subsequent eruption, according to GeoNet.

Prof Lindsay said it was possible 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."

Media playback is unsupported on your device Media caption How to speak volcano like a pro

Prof Kennedy noted that the volcano had been in a "state of heightened activity" for the past eight years and that "the type of volcano it is means small eruptions can happen at any time" with little to no warning.

But, he said, high levels of sulphur dioxide were the "tell-tale sign" that prompted the raised alert in November.

How big was the eruption?

Monday's eruption was classified as "moderate" on the volcanic alert system, according to GeoNet.

Prof Kennedy said this was because it only affected the crater area.

About 70% of the volcano is underwater, 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 some 47 people from around the world were on the White Island when it erupted on Monday. Thousands of tourists visit the volcano every year.

It 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.

Media playback is unsupported on your device Media caption Tourists could be seen waiting to be evacuated from White Island

Government agency GNS Science "put out their alert bulletins and have good communication with tour companies and they know what the risk is", Prof 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 eruption - an eruption driven by superheated steam and gas - are typically killed by "hot steam and water and what we call ballistic projectiles - blocks being thrown out of the vent", Prof Lindsay told the BBC.

Prof 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 Capt 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 has provided a backdrop to movies including the third instalment of The Chronicles of Narnia series.

"It's a spectacular, otherworldly place," said Prof 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."

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