The deepest part of the Indian Ocean is one of the least explored spots on Earth. It’s also one of the most dangerous. Major earthquakes have rocked it, causing major destruction—including what may be the deadliest natural disaster of the 21st century.

The Sunda Trench—also known as the Java Trench—is a gash in the ocean floor. It curves around the islands of Sumatra and Java, on the eastern edge of the Indian Ocean, between Australia and India. It’s about 2,000 miles long, and up to four and a half miles deep.

Only one expedition has studied the trench in detail. In 2019, both people and robotic vehicles descended to its floor. They found an abundance of life, including several new species. One highlight was a possible sea squirt—a critter that looked like a wrinkled balloon tied to a long string.

The Sunda Trench was created by the motions of the plates that make up Earth’s crust. Plates to the west are plunging below the plates to the east. The zone where they intersect forms a V-shaped hollow.

It’s an active zone—the motions of the plates trigger powerful earthquakes. A quake in 2004 caused a tsunami that killed a quarter of a million people around the Indian Ocean. In this century, only an earthquake in Haiti might have been deadlier. The event led to the creation of a tsunami warning system for the region—keeping a lookout for danger from the Sunda Trench.