Mud volcanoes can be found all around the world, and they're always pretty nasty. Popa Adrian/Shutterstock

Back in May of 2006, the world’s most disgusting volcano erupted on the Indonesian island of Java. Lusi, as it was named, didn’t erupt lava though – instead, sulfurous mud relentlessly poured out of the vent. Eleven years on, the eruption is still taking place, and it’s grown to be around 10 kilometers (about 6.2 miles) across.

As reported in the Journal of Geophysical Research: Solid Earth, a team of scientists led by the University of Oslo has finally solved the mystery of how Lusi formed. Despite not being a conventional volcano, it appears that it’s still driven by a magma chamber hiding nearby.

Seismic imaging systems have identified a complex series of faults that can be traced all the way from Lusi to Arjuno-Welirang, two conjoined stratovolcanoes. Although the sulfur-rich gas found in Lusi’s wet belches could also be found effusing from the nearby complex, no-one could until now prove that the two systems were definitively connected.

The magma has actually been cooking the organic-rich sediment layers directly overlying it. This generated plenty of gas, which got stuck beneath the surface.

Over time, the pressure built to a critical point, and then either a distant earthquake, or an accidental uncontrolled blowout in a nearby well, opened up the faults and triggered its explosive release. This eruption was far away enough from the magmatic system that, instead of carrying molten rock up to the surface, boiling mud was forced skywards instead.

At its peak, Lusi was erupting 180,000 cubic meters (6.4 million cubic feet) of sludge per day, some of which ended up destroying 60,000 homes and killing several people in the process.

The constant heat source means that the organic matter is continuously being baked; consequently, Lusi will continue to effuse the pungent mud for the foreseeable future. Mud continues to flow from the vent at a rate of around 32 Olympic-sized swimming pools per day, and villages have since been buried 40 meters (130 feet) beneath the surface.

Despite this discovery, Lusi remains enigmatic. After all, scientists can’t quite agree on what Lusi even is.

True mud volcanoes are often no larger than a room and can be found all over the world, from Iceland to Indonesia to North America. Many of them are created when mud beneath the surface is put under pressure and is squeezed onto the surface like toothpaste.

Lusi is a hybrid, in that it’s part-mud volcano, and part-hydrothermal vent, the boiling water-emitting chimneys you normally find on the seafloor. As a result of its weird fueling mechanism, Lusi is more akin to a geyser – just one that’s less majestic and far grosser.