Not long ago gas prices hit an all-time high in the area where I live. My brother-in-law, visiting this country for a family wedding, watched as I filled my car's tank and expressed amazement at how cheap petrol was in the United States. In England, he pays at least three times as much per gallon.

He also saw the size of some of our gargantuan SUV's, and he wondered: Doesn't anybody here worry about running out of oil? It's a good question.

On the one hand, we've been hearing horror stories for 30 years or more about what will happen when we deplete the global supply of fossil fuels. On the other hand, the estimates of fuel reserves today seem just about the same as they were a generation back. While on the third hand, if I had one, since fossil fuels are not being replaced - otherwise they couldn't be fossil fuels - at some point in time we absolutely must run out of them.

Lots of factors are at work here, and together they build up to a confusing picture. At the moment, oil provides more of the world's generated energy than any other source, something between 35 and 40 percent. And although we are gobbling up oil, active exploration has so far pretty much kept pace with use. Just a week or two ago we heard about a new giant field in Kazakhstan, north of the Caspian Sea.

If the price of oil goes much higher, other forms of petroleum also become economically attractive. Oil shales and oil tar sands are largely unexploited, and some of them hold monstrous reserves. The Athabasca tar sands, to take one example, contains at least five times as much oil as Alaska's Prudhoe Bay, and that field is in the world's top ten producers.

Let's assume that discovery can't keep up at its present rate. Suppose that we begin to run out of oil half a century from now. What then?

Well, there is coal, which worldwide is still the second-largest producer of energy, a bit less than 30 percent of the total. The reserves are generous. This country has somewhere between 100 and 1,000 thousand years worth of coal, at current use rates. Of course, we have some problems with coal, since OSHA won't let you mine it, and EPA won't let you burn it. Coal is also a widely variable material, ranging from high-energy low-ash anthracite to dirty and sulfurous lignite. But assuming that we can clean up coal enough to burn it without poisoning ourselves, we are good for at least a century.

And after that?

Well, we have not yet considered natural gas, which today provides close to 20 percent of the world's energy. How many years of reserves of natural gas do we have? Here, at last, we are reaching the borderlands of today's knowledge. Natural gas (so-called to distinguish it from "producer gas," which humans make from coal or coke) is almost all methane, the simplest of the hydrocarbons. Methane's molecule consists of one atom of carbon and four of hydrogen. It is natural in another sense, being produced in large quantities by decaying vegetable matter ("marsh gas") and the digestive processes of cattle.

Since these sources of methane are difficult to harness for energy production, we would like to know how much "fossil gas" there is. And we don't know. Some scientists believe that most of the natural gas close to the surface of the earth has been there since the formation of the planet, and comes welling up as "primordial" methane. If this is true, the potential reserves could be enormous.

Much less conjectural is the fact that unknown amounts of methane are locked up in what are known as methane clathrates. "Clathrate" is an odd word, which comes from the Latin word for a cage, and that's exactly what a methane clathrate is: four molecules of methane, locked into a "cage" made up of 23 molecules of water.

There are two things to note about a methane clathrate. First, at the temperatures found deep in the ocean, it is solid, not liquid; and second, it is dense enough so that, unlike ordinary water ice, if it forms on the ocean bed it will not float up to the surface.

Everything that sinks down from the surface of the sea decays and rots, so that even without the unknown factor of upwelling primordial methane, the sea-floor contains a great deal of methane. And there is of course abundant water. As a result the ocean floors are believed to hold vast clathrate beds, hundreds of miles across and 50 feet or more thick. If we mine these clathrates and bring them up from the depths, they will automatically release their methane in the higher temperatures at the surface. We will have natural gas, in unprecedented abundance.

Maybe a century from now the word "mining" will not bring to mind an image of someone tunneling within the earth. Our miners of the future will be explorers of the deep sea. Our use rate of fossil fuels may be more than ever, but our estimates of future fuel reserves, because of sea-floor methane, will be higher than they are today.

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