In 1870, explorers stood in awe as Mud Volcano spewed mud into the treetops and shook the ground with each eruption, but two years later it had become a pool of bubbling, muddy water after blowing itself apart. Nathaniel P. Langford recalled hearing dull, thundering sounds, likened by General Washburn to frequent discharges of a distant mortar, and following them to a mud volcano on the slope of a small hill in a grove of pines. Dense volumes of steam shot into the air through a crater thirty feet in diameter, with irregular reports occurring as often as every five seconds, audible for half a mile, and every other report shaking the ground for two hundred yards or more while massive jets of vapor burst forth like the smoke of burning gunpowder. The rotten-egg smell comes from hydrogen sulfide gas rising from Yellowstone's magma chamber. Thermophiles use this gas as a source of energy and help turn it into sulfuric acid, which breaks down rock and soil into mud. Many of the visible colors come from vast communities of thermophiles, some are the yellow of pure sulphur, and when iron mixes with sulphur to form iron sulfide, gray and black swirls sometimes appear in the mud. For many decades, visitors have been intrigued by Mud Volcano's powerful odor and the yellow sulphur most common in late summer.