The volcanoes are so large that you can only see a small part of them at any one time," says Charlie Paull of the Monterey Bay Aquarium Research Institute in Moss Landing, California, US. "They erupt frequently and release gas and mud, which bubbles out onto the seafloor. The mud has a viscous texture like the top of a very thick stew.
The eruptions contain many different chemicals. First on the list is methane, otherwise known as natural gas.
"The gas coming out of the volcanoes is biogenic methane, which means that it is produced by the decomposition of organic matter and bacteria, and not from oil deposits trapped underground," says Paull.
A lot of Earth's methane is trapped below the seabed as "gas hydrate", an ice-like crystal of methane and water. Hydrates form when the intense pressures at depths of over 500m freeze the methane and water. When mud volcanoes erupt near gas hydrates, the methane in the hydrates can escape, along with methane from deep underground.
The world's mud volcanoes are estimated to release 27 million tonnes of methane every year, about 5% of annual global emissions.