The bubbles of the Bledug Kuwu geyser grow up to 30 feet high before they suddenly explode, sending shreds of mud flying through the air and releasing a white cloud of carbon dioxide. Strictly speaking, Bledug Kuwu is not a geyser, which is by definition powered by overheated ground water and blows out fountains of hot steam. The driving force of this mud geyser is upwelling volcanic carbon dioxide, although its exact mechanism is not yet clear.
The ground near the mud geyser is springy under foot but, if you get too close, you will slowly sink into the lukewarm, grey mass. Apart from carbon dioxide, the geyser also ejects mineral water, particularly in the rainy season, which local villagers boil down to a sweetish salt they can sell at a high price.
Photo credit: Bernhard Edmaier