HUBBERT'S PEAK IS

not a mountain, but it's just as good at hiding whatever's on the other side. One anonymous day in the not-too-distant future it will be the most crowded place on Earth. And coming down won't be nearly as much fun as going up.

Hubbert's Peak is named for M. King Hubbert, a prickly but brilliant Shell geologist who went to a meeting of his colleagues from the petroleum industry in 1956 and told them that they had 15 good years left, at most, before U.S. oil production peaked and began a rapid decline.

Hubbert's paper marshaled the historical record of mature U.S. deposits, mathematical modeling and geological facts to posit a distinctive bell curve for the available reserves of fossil fuel on any scale, with exponential growth on the left as extraction ramps up, and a symmetrical decline on the right as the dinosaur mush is depleted.

Hubbert then showed how the right shoulder of the curve could be stretched with improved technologies and additional deposit finds. But the basic premise didn't change: It would take us decades, rather than centuries, to exhaust the bulk of the energy stored within the Earth over the preceding 500 million years.

The paper made waves and turned Hubbert into a figure of controversy, but proved eerily precise in pegging the U.S. peak. So when the price of a crude barrel shot above $70 last year, more than a few people recalled Hubbert's prediction that "the culmination of world production of [oil and natural gas] should occur within about half a century."

Coincidentally, global output has been basically flat for the last two years. Whether this is Hubbert hitting another home run or not is hard to tell. But there's no denying the fact that global discoveries of oil and gas peaked in the 1960s, or that discoveries have been lagging behind output for the last 20 years.

The giant Cantarell oil field accounting for half of Mexico's oil production endured a 12% drop in output last year, and is expected to decline another 15% in 2007. Meanwhile, Norway's Statoil replaced only 73% of the reserves it used up last year, and said its 2007 production target could prove hard to meet. "All the easy barrels are gone or out of reach," explained the CEO this week.

Mexico's troubles are the product of chronic underinvestment and mismanagement by a state monopoly that shuts out foreign investment. Iran and Venezuela have the same problems, only more so.

The head of exploration and production for Chevrontold an industry conference this week that how much oil's in the ground is not the real limiting factor. "The truth is we could still run short of oil, above ground where access and politics come into play," he was quoted as saying in the Houston Chronicle.

He was, incidentally, something of a wild-eyed futurist in his day and, later, a bitter critic But the battle between his followers and the establishment grinds on.

The establishment includes Saudi Arabia, which claims it could pump oil at the current rate for over a century, and Exxon Mobil, which has run ads suggesting you still need binoculars to gaze at Hubbert's Peak. "The good news is that abundant oil resources are available to meet the projected growth in demand," said Exxon's boss this week. He thinks the world might have three trillion barrels of oil left, three times more than has been used to date and more than twice the remaining reserves Hubbert's model would postulate.

Also on the "cornucopian" side of the debate is Cambridge Energy Research Associates, which published its own upbeat estimate last fall as part of the broadside against "peak oil" adherents. While many "peakists" predict a decline in output within a decade at most, CERA forecasts no peak until 2030, to be followed by an "undulating plateau," which sounds like a pleasant place to graze.

CERA's critique was treated by the scarce-oil camp as a toxic spill, drawing stinging rebuttals. Hubbert's disciples have also been busy writing books. And whether their criticism of official reserve estimates proves merited or not, it's not as if too many pros take the extreme case for abundance at face value. For instance, IHS Energy, owned by the same corporate parent as CERA, estimates Kuwait's remaining reserves at perhaps half the official number.

Meanwhile, the energy markets muddle along, preoccupied with next week's weather rather than next year's exploration and production. A warm winter and abundant U.S. stockpiles have induced OPEC to cut export quotas, successfully propping up crude prices. China and India keep growing. Exploration and production costs keep going up. Getting to Hubbert's Peak promises to be much more than half the fun.