Sounding out burning snowballs.

Measurements of sound waves traveling through seafloor sediments are gradually enabling researchers to determine the distribution of a peculiar substance known as methane hydrate, which consists largely of high concentrations of methane gas trapped, or dissolved, in ice. Found in deep-sea sediments throughout the world, this material contains so much methane that a flame can readily ignite a chunk brought to the surface.

Researchers are interested in methane hydrate because it poses a potential hazard when drilling for and extracting petroleum -- especially as oil exploration heads for deeper waters. For example, hot liquid brought up from great depths could melt the hydrate, destabilizing the platform used to produce the petroleum.

If released to the atmosphere, these stores of methane could also contribute to global warming. At the same time, the deposits may represent a possible source of energy. Recent acoustic measurements indicate that one area off the coast of South Carolina -- roughly the size of the state of Maryland -- may contain enough methane to supply the United States for 350 years at its present rate of consumption.

"We need to find out where it is and how much there is, and about the only way we have access to this substance right now is through acoustic measurements," says oceanographer Aubrey L. Anderson of Texas A&M University in College Station. "Very few samples have been taken." But interpreting acoustic signals from the seabed remains an inexact science. Anderson chaired a special workshop devoted to a general discussion of how to improve field measurements of "gassy" seafloor sediments.