A School of Mud Volcano Islands in Azerbaijan

Satellite view of a tadpole-shaped brown land area encircled by blue-green water.

Long spits of muddy sediment are visible behind islands created by mud volcanoes in an image captured on August 30, 2025, by the OLI (Operational Land Imager) on Landsat 8.

NASA Earth Observatory/Lauren Dauphin

Today’s story is the answer to the April 2026 puzzler.

With its abundance of naturally occurring gas seeps and fires, Azerbaijan has long been called “the land of fire.” Yet burning mountains are just one of the geologic wonders found in the small Eurasian country on the Caspian Sea.

Azerbaijan is also home to at least 220 mud volcanoes, according to data from the Azerbaijani government, though some researchers put the total number closer to 350. That is thought to be one of the highest concentrations of mud volcanoes on Earth.

Mud volcanoes—as well as gas seeps—are found within sedimentary basins where geologic conditions have allowed hydrocarbons to accumulate. Such basins typically have fluids and gases, such as oil and methane, trapped beneath sedimentary rocks and under high pressure. Instead of erupting molten lava, mud volcanoes typically eject cold slurries of mud, water, methane, and other gases. Oil and gas form from the remains of marine organisms, such as phytoplankton and algae, which settle on the ocean floor and are later transformed by pressure and heat.

Many of Azerbaijan’s mud volcanoes are clustered near the cities of Baku and Qobustan on the Absheron Peninsula, an area where structural folds and faults in the landscape have created cracks that allow methane-rich mud to move up toward the surface. On land, mud volcanoes typically form conical structures anywhere from 20 to 400 meters (70 to 1,300 feet) tall and 100 to 4,500 meters in diameter.

There are also at least 140 underwater mud volcanoes in the South Caspian Sea along Azerbaijan’s coast, including eight islands in the Baku archipelago. The satellite image above shows one of them, the tadpole-shaped Xərə Zirə Adası (also known in Russian as Ostrov Bulla), which had violent eruptions in 1961 and 1995 and still has two “weakly active” mud volcano vents, said Adelaide University geologist Mark Tingay. The neighboring island to the northwest, Duvannı (Ostrov Duvannyy), is visible in the wide view below. It erupted in 2006 and still has active vents on its northern side.

“The islands’ ‘tails’ are most likely caused by currents eroding their weak mud deposits,” Tingay said. “They look like spits of eroded and redeposited sediment that formed on the lee of the island, where current and wave action have the least effect.”

Four tadpole-shaped mud volcano islands are visible along the Caspian Sea in this image captured on August 30, 2025, by the OLI (Operational Land Imager) on Landsat 8.

NASA Earth Observatory/Lauren Dauphin

There are two more tadpole-shaped islands to the south, with sediment “tails” also oriented to the southwest. One of these—Səngi Muğan Adası (Ostrov Svinoy)—is known for producing particularly violent eruptions, most recently in 2002 and 2008, Tingay said. One of its most notorious events occurred in 1932 when, without warning, it released a 150-meter-tall fireball in an eruption that caused 13 injuries and almost destroyed the island’s lighthouse, he added.

Though mud volcanoes are interesting to geologists and often indicators of underground fossil fuels, they can be unpredictable and pose risks. “They have the potential for ‘paroxysmal eruptions’—short but extremely violent eruptions,” Tingay said. “They sometimes fuel huge fireballs and have created whole new islands in the space of a few minutes.”

NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey. Story by Adam Voiland.