Volcano popular with tourists erupts in northern New Zealand leaving at least five dead and many trapped.

New Zealand‘s most active volcano erupted violently on Monday with at least five people killed and many still trapped on White Island with rescuers unable to reach them.

Here is all you need to know about the deadly volcano eruption:

What is the island volcano?

The volcano on New Zealand’s White Island, a popular tourist destination, is the most active in the country. About 70 percent of the volcano is under the sea and at least 10,000 people go to see it every year.

The White Island, also known by its Maori name Whakaari, is about 50 kilometres (30 miles) from the east coast of North Island.

The volcano’s last fatal eruption was in 1914 when it killed 12 sulphur miners. White Island became a private scenic reserve in 1953.

What happened?

On Monday at about 2pm local time (01:00 GMT), the volcano erupted killing at least five people and leaving an unknown number still stranded on the tourist island.

GeoNet agency, which had been monitoring the rumbling volcano, raised its alert level to four out of a maximum five.

The “short-lived eruption” threw an ash plume about 3,658 metres (12,000 feet) high, New Zealand’s geoscience agency GNS Science said in a statement, adding that there were no current signs of an escalation.

The eruptions also sent debris into the air. Video emerged of some visitors stuck in a crater, while rescue operations were on hold until conditions became safe.

The images showed smoke first engulfing the top of the crater and then the entire island.

Emergency services attend to an injured person arriving at the Whakatane Airfield after the eruption [Alan Gibson/New Zealand Herald via AP]

Who was on the island?

Police initially said there were 100 people on or near the island, then later revised the number down to 50.

Prime Minister Jacinda Ardern said there were “a number” of tourists in the vicinity, both from New Zealand and overseas. Those injured – numbering about 20 – suffered burns.

Police said the number of missing people on the island was “in double digits”.

Was the eruption expected?

GeoNet warned on December 3 that “the volcano may be entering a period where eruptive activity is more likely than normal”.

But the geological hazard monitoring website added: “The current level of activity does not pose a direct hazard to visitors.”

The eruption was unexpected but not unusual, an expert said.

“Sudden, unheralded eruptions from volcanoes such as White Island can be expected at any time,” Shane Cronin, a volcanologist at the University of Auckland, said in comments published by the Australia Science Media Centre.

“We know hydrothermal and so-called ‘phreatic’ eruptions can occur suddenly and with little or no warning because they are driven by the expansion of super-heated water into steam,” he added.

Several eruptions have taken place on White Island over the years. The most recent in 2016 left no one hurt.