In 1956, U.S. geophysicist Marion King Hubbert earned scorn and contempt for suggesting that the world's oil supply was not only finite but would start to dry up around the end of the 20th century.

Chicken Little that he was, Hubbert also predicted that as soon as 1970, the rich fields of Texas and those elsewhere in the lower 48 states would fail to meet our demand.

Then the U.S. home-produced supply did start to dry up in the 1970s. Now, we're pretty much out of all the easily accessible oil. That's why companies are going a mile underwater and drilling thousands more feet into the seafloor to get at high-pressure oil deposits full of potentially explosive natural gas.

In retrospect, it sounds somewhat dangerous.

Last week, President Obama put a six-month moratorium on "ultra-deep" drilling after yet another failed attempt to stop the Deepwater Horizon leak from hemorrhaging about 800,000 gallons of oil a day into the Gulf of Mexico. A cap now seems to be stopping some of the oil.

But unfortunately, those "ultra-deep" oil deposits are pretty much all that's left, said geophysicist Roger Anderson of Columbia University's Lamont-Doherty Earth Observatory. "This is our last big gasp."

Anderson thinks the leaking BP deposit is so enormous that the company will still make a fat profit even after attempting to clean up the mess. "It's going to be a billion barrels plus," he said.

Until the 1990s, nobody even knew that gigantic deposits of oil and natural gas had pooled under the deepest parts of the ocean.

Those deep deposits got there thanks to the unique geological history of the Gulf of Mexico - a body of water that started to form around 60 million years ago.

Over time, Anderson said, silt from the Mississippi River would periodically close off parts of the gulf, starving the region of oxygen. Those periods of oxygen starvation are key to the production of oil. That's when dead things - mostly plankton - get buried in the seafloor without decomposing.

When there are no bacteria to extract energy from all this dead stuff by eating it, the organic matter is cooked into oil, said Anderson, and we extract the energy by burning it.

Unfortunately, the dynamic history of the gulf also created vast salt sheets on the seafloor, and these add another element of danger to oil extraction.

The salt sheets formed as the gulf periodically dried up over the eons. Slowly, the sheets hardened and were folded and buckled by tectonic forces, creating a jagged salt-scape where pockets of oil and gas get trapped. Because the salt is so impermeable, it can seal up gas and oil at dangerously high pressures.

When you drill into the salt and start extracting the oil, the gas can suddenly come bubbling out, like the fizz in a soda. "If you go down there drilling, it's like poking a hole in a balloon," said David Scholl, an emeritus geologist with the United States Geological Survey.

It's the same thing that gives volcanoes their explosive force - huge quantities of gas can dissolve in molten rock at high pressure, but once it starts moving to the surface, the gas bubbles out, often with violent consequences.

Columbia's Anderson said many of the gulf's ultra-deep wells are full of gas. They even have a name for the dangerous bubbles that can come up the pipes: "Kicks." Any time you stick a new pipe in a deposit, "it's unknown what you're going to find," he said.

That's why he was shocked that BP wasn't better prepared. "In my career of 30 years, I've never heard of a blowout preventer failing," he said, referring to the device that was supposed to close off the leaking pipe.

He compared the company's lack of preparation to that of an airline that hasn't considered the possibility of a crash.

In theory, he said, this could have been done safely.

But humans tend to do things unsafely, especially if it's cheaper. And even the ultra-deep deposits will last only a few more years in a world that's burning through more than 80 million barrels of oil a day.

In the 2004 book Out of Gas, California Institute of Technology physicist David Goodstein describes a worst-case scenario in which Americans of the 21st century replace diminishing oil with coal, perhaps even liquefying it into fuel in an inherently expensive and dirty process.

Little did we know how much worse it could get.

Contact staff writer Faye Flam at 215-854-4977 or fflam@phillynews.com.