It looks like a dirty snowball. It fizzles and pops when left at room temperature. If you saw a chunk of it on the street, you would probably boot it aside like a piece of unruly ice.
Yet this peculiar substance has enough energy bound up in it to stoke the nation's furnaces for more than a hundred years - and is thus drawing increasing interest from scientists in the United States and around the world.
The source: methane hydrate - solidified natural gas that is, in effect, fuel-soaked ice found in the ocean bed and under the permafrost of such areas as the Arctic and Alaska. Scientists have long known of its existence, but only recently have begun to probe its properties and the ways in which it might be tapped.
Enough technical snags remain that some researchers doubt it will ever be used to fuel factories or heat homes - certainly not before the 21st century. But the energy exists in such vast quantities that it might not be possible to ignore it in planning for a post-oil era.
The best US estimates put worldwide deposits of hydrate-bound natural gas at 1,100 trillion cubic feet (TCF) on land and roughly 1 million TCF offshore. In the US, the amount is believed to be 23 TCF on land and 2,700 TCF within US territorial waters.
By comparison, US ''proven reserves'' of natural gas add up to 190 TCF - about a 10-year supply at present consumption rates. So, based on current usage, there is enough methane hydrate in US land and waters alone to extend present natural gas supplies by more than 150 years.
''Even by the most conservative estimates, it's a big energy source,'' says E. Dendy Sloan, professor of chemical engineering at the Colorado School of Mines and an expert on methane hydrate.
Methane hydrate is a mixture of about 85 percent water plus methane (natural gas). It is formed under high pressure at cold temperatures. The result is a grayish-green ice ball that holds energy in a highly concentrated form: One part methane hydrate contains the equivalent of 170 parts regular natural gas.
One reason so little was known about the substance until recently is that there hasn't been any of it around to study. Two years ago, the research vessel Glomar Challenger drove a steel probe into the ocean floor off the Guatemalan coast. It brought back what are believed to be the West's only hydrate samples from beneath the sea.
Given their scarcity, the pieces are coveted like moon rocks in the research community. Hydrate chunks have been distributed to half a dozen universities and other research centers across the US. The main repository is at the School of Mines in Golden, Colo., under the eye of Dr. Sloan, a bespectacled, bow-tie-wearing scientist, who puts the replacement value of the overall stock at $1 million.
The snowball-like samples are kept in a steel container, refrigerated at -40 degrees F. At room temperatures, the chunks smoke and pop - the result of gas escaping. If kept out too long, they disintegrate.
It is some of these properties that make such ''energy-dipped snow cones'' so uncertain as a fuel for tomorrow's furnaces. Scientists aren't yet sure it even can be extracted. Part of the problem is that it's a solid, which means it can't be retrieved from the seabed or permafrost with conventional drilling techniques.
The first attempts, if there are to be any, would likely be on land-based deposits, possibly in Alaska. Deposits are known to exist there, and a pipeline distribution system is already in place.
To tap this energy, however, would probably require putting a heat source down the drill hole, either in form of a hot fluid or controlled fire. This would melt the ice-rock and allow it to be drawn up through conventional wells. But this would be exorbitantly expensive and technically tough. The Soviets, leaders in hydrate research, have had some success pumping antifreeze down a well.
Some land-based deposits are close enough to the surface that they could be mined. But here again lurk problems of controlling the temperature and preventing gas from escaping.
''Methane hydrate is truly an unconventional source, beyond anything we have encountered,'' says Charles Komar, head of unconventional gas projects for the US Department of Energy, which this year will spend $1 million on hydrate research. ''Nobody really knows whether it is technically recoverable yet.''
With all the uncertainties, bottom-line-minded oil companies are not queuing up to probe this new frontier. Big oil is familiar with it. It has long had to cope with hydrates clogging gas pipelines: If water vapor is present in high-pressure lines, hydrates often accumulate. Many firms are studying how to build hydrate-free distribution and production facilities - not how to use hydrate as energy.
''The issue is hydrate at what price,'' says an Exxon official, who sees plenty of conventional gas yet to be found. ''The current reading is that for the foreseeable future'' it's not economical.
Even Komar believes other synthetic fuels will be tapped first. But Dr. Sloan believes methane hydrate will be part of the US energy mix in the next century.