In the latest natural disaster to strike Indonesia this year, at least 429 people have died after an eruption at one of the archipelago’s volcanoes is thought to have triggered a tsunami.

At least 1,459 people have been injured and dozens are reported missing after the disaster struck without warning in the middle of the night during the busy pre-Christmas weekend. The main affected areas are the coastlines of western Java and the southern Sumatra islands, the Associated Press reports. It follows a devastating 7.5-magnitude earthquake and tsunami that hit the region of Sulawesi in September, killing more than 2,100. Here’s what to know about the latest tragedy.

What Caused the Disaster?

The eruption of the Anak Krakatau volcano, the name of which translates to “Child of Krakatoa,” on Saturday night local time is thought to have triggered the devastating tsunami the following day. After one of the biggest ever-recorded eruptions more than a century ago, a new volcanic island emerged in Indonesia’s Sunda Strait.

Scientists, including those from Indonesia’s Meteorology and Geophysics agency, are continuing to analyze why the tsunami happened, but have pointed to undersea landslides following the volcanic eruption as a possible root cause. “Over the last month, Krakatoa has been increasingly active again and the activity we have been seeing was pretty normal for this volcano,” says Dr. Janine Krippner, Volcanologist at Concord University. “It looks like a case of the tsunami was a partial flank collapse at Krakatoa, which is where part of the material of the volcano collapses into the sea.”

“Here, due to the collapse of part of the volcano, the magma is interacting with the ocean, causing the water to steam and expand significantly in volume and adding more explosive power. We can also see some dark grey and brown in there, which is the volcanic ash erupting from these explosions,” Krippner tells TIME, analyzing the above photographs. “We also have another process occurring here as the billowing plumes of white and brown sea water and ash look like it is forming a pyroclastic surge along the water. It is quite dangerous, especially as there is no way for us to know if there will be any more collapses or any more tsunamis caused by these collapses. We just don’t know how unstable the volcano is.”

That caution has been echoed by Indonesia’s National Disaster Mitigation Agency, who have warned that as the volcano continues to erupt, the possibility remains that another tsunami could be imminent. “We are cautioning the people to remain cautious,” said a spokesperson for the agency Sunday, according to CNN.

Captured on Video

Dramatic videos show fire and smoke rising into the sky on Saturday morning and lava flowing down the steep slopes of the Krakatoa volcano, the day before the tsunami hit.

Other striking videos and photographs have circulated on social media showing the scale and extent of the devastation, as well as the moment the tsunami hit. Members of Jakarta pop band Seventeen were performing a concert at the Tanjung Lesong resort in western Java in when a powerful wave struck, smashing through the stage, reportedly killing the band’s bassist and manager.

Who Is Responding?

Indonesia’s national disaster agency said that the worst-affected area was the Pandeglang region of Java’s Banten province, an area popular with tourists and encompassing Ujung Kulon National Park and popular beaches. The country’s President Joko Widodo expressed his condolences and ordered government agencies to respond to the devastation.

Indonesia has also received expressions of support from around the world, including from U.S. President Donald Trump, who tweeted that “we are praying for recovery and healing.”

“The United States conveys our deepest condolences for the loss of life and destruction from the tsunami that struck West Java and southern Sumatra, Indonesia, on December 22,” a spokesperson for the U.S. Department of State said. “Our thoughts and prayers are with all those in Indonesia affected by the natural disaster. We are currently not aware of any U.S. citizens directly affected, but stand ready to assist as needed.”

A spokesperson for UNICEF told TIME that the organization was “deeply saddened and concerned” by the impact of the tsunami, adding that “whereas we stand ready to assist, preliminary reports indicate that local government and organizations on the ground have adequate capacity to respond. Should the situation change and assistance is requested, UNICEF USA is ready to help the most vulnerable children in affected areas.”

Doctors Without Borders has mobilized medical teams to support health centers in Carita and Labuan, both in Pandeglang regency, as well as a mobile team to treat injured patients that are unable to reach health facilities.

A String of Disasters

Indonesia is highly susceptible to natural disasters given its location along the “Ring of Fire,” an arc of volcanoes and fault lines surrounding the Pacific Basin. “There are 127 active volcanoes in Indonesia, and so it is very normal to have more than one volcano producing an actual eruption at any one time,” Krippner tells TIME. “But what we are seeing here is one of the worst case scenarios of this particular volcano, because of the interaction with the ocean.”

In August this year, a quake on the island of Lombok killed more than 500 people. The following month, a devastating tsunami and earthquake struck Sulawesi, killing thousands and swallowing entire neighborhoods through the phenomenon of liquefaction.

Speaking about the latest disaster, Krippner says that the lack of warning was not the result of negligence on the part of the Indonesian government. “In this kind of tsunami that has been triggered by a volcano, a warning system is not really viable as the location of the volcano and the area impacted by the tsunami are so close to each other. Usually, there is enough time to pick up on the warning, but unfortunately in this case, there was just not enough time.”

Get The Brief. Sign up to receive the top stories you need to know right now. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.