RIGHT: When the average car drove 13.4 miles on a gallon of gas, gas lines formed after the 1973 OPEC oil embargo. In Portland, Ore., each driver could buy only five gallons at a crack.

Courtesy National Archives and Records Administration.

The hydrate stability zone exists within a range of pressure and temperature. At high pressure, hydrate stays solid above 273 degrees K, or 0 C, the normal freezing point of water.

Courtesy the University of California, Lawrence Livermore National Laboratory, and U.S. Department of Energy.

These polychaete worms – "ice worms" to their friends – have a symbiotic relationship with bacteria that metabolize methane from methane seeps on the floor of the Gulf of Mexico.

U.S. Department of Energy

The Glomar Challenger, built in the 1960s, was the first research ship to take core samples from the deep ocean floor.

Courtesy DOE

Lotta gas about cold gas
Gas hydrates are solids -- one of the five "states," or "phases," of matter. Everybody knows about gases, liquids and solids. A fourth state, plasma, consists of electrically charged particles and is found in fluorescent lights and the sun. A fifth state of matter, the recently discovered Bose-Einstein condensate, exists near absolute zero.

Gas hydrates contain a matrix of frozen water surrounding molecules of natural gas. Why are they found at high pressure and low temperature? Let's start with temperature: when temperature falls, liquids and gases tend to crystallize - AKA freeze. Their molecules vibrate more slowly, and since vibration is what causes fluids to flow and take the shape of their container rather than act like a solid block of ice or table salt, the removal of thermal energy allows most fluids to freeze into a crystalline structure.

The temperature of crystallization is also related to pressure: At high pressure, warmer fluids can freeze. The reasoning is deeply rooted in impenetrable thermodynamics, but essentially, higher pressure tends to "push" molecules into the crystalline structure, while lower pressure allows the molecules to move more rapidly – to be a liquid or gas instead of a solid.

Confused? Think about the transition from liquid to gas. If you lower the pressure, liquids boil (vaporize) at a cooler temperature because there's less pressure "pushing" the molecules into the liquid phase. Because boiling takes place at a lower temperature at high altitude, you must boil food longer than usual.

Gas hydrate depends on cold conditions, and the lower boundary of a deposit is determined by the steady increase in temperature with depth - caused by the extreme temperatures inside the Earth.

Biologically speaking
In an era when organisms are being found in ever-more hostile environments, hydrates raise some interesting biological questions. While hydrate in the Gulf of Mexico probably comes from thermal decomposition of hydrocarbons, most hydrate elsewhere seems to reflect bacterial decay of organic crud.

It doesn't take a genius to realize that bacterial decay requires bacteria. But these bacteria must live in places without sunlight and oxygen - places until recently considered inhospitable to life. In recent years, of course, biologists have discovered the "hot, deep biosphere," whose inhabitants do the decomposing that results in much gas hydrate. Gas hydrate offer some treats to the naked eye (and the naked nose, too, for that matter). Jean Whelan, a researcher at the Woods Hole Oceanographic Institute, says that samples brought up from the hydrate zone "are very tangled up with biology."

Tube worms eat the hydrate and live inside it, she adds. "It would not surprise me if hydrates were forming and disintegrating simultaneously, in a steady-state sort of thing. There are little worms living on it, are obviously adapted to it."

The samples also "stink to high heaven," she adds, with the clinging odor of mercaptans - the sulfur-bearing compounds that, for safety purposes, are put into natural gas before it's distributed in city pipelines. The characteristic stench of mercaptans are another indication that the methane in gas hydrate comes from anaerobic decomposition.

Down discovery lane
If methane hydrate deposits are so widespread, why weren't they noticed during the first century of oil and gas production? Because drillers went for the easy petroleum, starting on land, in temperate or tropical climates. Even when drillers moved to the ocean, they emphasized, naturally enough, shallow water. Only relatively recently have they drilled in permafrost and deep water.

Dillon of the USGS notes that gas hydrates were not discovered in nature until 1970, in drill cores from the Deep Sea Drilling project. (Hydrates did occur as stubborn plugs in cold-region natural gas pipelines during the early 1900s.)

The drill cores bubbled and fizzed like a submarine version of a freshly opened soda bottle. "People on the ship didn't know what to make of them," says Dillon. "It was not until afterwards, back in the lab, that people thought about them and realized gas hydrates existed in seafloor sediment."

An echo of science
Most of what we know about the abundance of gas hydrates comes not from drill holes but from reflected seismic waves. Earth is a complicated reflector of sound, and when you set off dynamite - or make a tooth-jarring clamor with another means - the reflection pattern paints a picture of subsurface conditions.

The results are open to interpretation, but seismic techniques are a darn sight cheaper than drilling hundreds of holes to figure out what's downstairs, so the technique has been adopted by seismologists, who also use earthquake waves to explore the large-scale structure of the Earth.

Generally, the seismic signature of hydrate comes from the bottom boundary of the hydrate zone, where gradual warming causes a transition between hydrate above and free gas below. But Holbrook says his research group, now working in the Atlantic, has detected hydrate directly in seismic soundings. "One of the exciting things we've seen is a clear reflection from a hydrate layer. This may be the first time that anyone has seen a reflection that's indisputably caused by hydrates."

Such a technique would go a long way to proving whether the astonishing resource estimates are true.

Let's say they're accurate. How to get this stuff out of the ice and into the furnace?