Washington ? To meet the world’s boundless thirst for oil, drillers are searching in the sand and mud of remote western Canada, the tough shale rock of North Dakota and more than a mile under the seas off the southern U.S. coast, where a drilling accident has sent hundreds of thousands of gallons of crude spewing into the Gulf of Mexico.

Why are we going nearly to the ends of the earth and the bottom of the seas for oil?

The answer, say many experts, is that we’re consuming as much oil as we ever have but the era of “easy oil” is in our rearview mirror and receding fast.

Production from onshore oilfields in the U.S. has been declining since the 1970s, and near-shore production along the Gulf of Mexico peaked more than a decade ago. Many of the richest remaining conventional deposits are in places that are politically unstable, such as Iraq and Nigeria, or hostile to Western oil companies, such as Sudan, Venezuela and the Middle East.

The Development Driller III, which is drilling a relief well, is seen at sunrise from the Joe Griffin supply boat at the site of the Deepwater Horizon oil spill containment efforts Saturday in the Gulf of Mexico off the coast of Louisiana.

While Americans remain tethered to a petro-driven economy and surging demand from China and other emerging markets is driving up global demand, the quest for new sources requires more money and technological wizardry than ever before. As anyone tracking the massive Gulf spill can attest, it brings greater risks as well.

“No one goes and tries to drill in a mile of water if they can think of somewhere easier to do it,” said Chris Skrebowski, a former strategist for British Petroleum who now runs a London consultancy that studies oil depletion.

“The easy stuff that you have access to … is already spoken for. All that’s left is the frontiers, which are necessarily more technically challenging.”

Just as the space program pushed its horizons farther and farther away in the last century, occasionally suffering devastating setbacks, the 21st-century search for oil is testing the limits of science and the environment. It also presents the Obama administration and Congress with a policy problem to which there’s no easy solution.

Weaning the U.S. off oil has never been politically convenient, and it’s even less so with the nation slowly climbing out of a deep recession. The most promising approaches include sharply higher gasoline taxes and mileage standards and increased use of nuclear power, wind, solar energy or geothermal power — all of which have their own drawbacks.

Pushing ahead with unconventional drilling in the wake of a major spill could seem risky, but putting the brakes on exploration would worsen what analysts warn is an impending oil price crunch as world demand increases and production slows.

“An oil spill here or there hasn’t gotten in the way of oil extraction anywhere,” said Peter Maass, the author of the 2009 book “Crude World: The Violent Twilight of Oil.” “We want our oil, and we’re pretty much willing to pay any price for it.”

During the past decade, Americans have curbed per capita oil consumption slightly. Overall U.S. demand is roughly the same as it was in 2000, when the population was about 7 percent smaller, according to official statistics. However, with China and India together adding more than 1.2 million cars each month, according to figures from the Organization for Economic Cooperation and Development, there’s more global competition for oil than ever before.

The U.S. still imports more than half the oil it consumes, and energy companies are racing to shore up the dwindling supply from conventional sources.

Before the Deepwater Horizon offshore rig exploded and sank late last month in the waters off Louisiana, drilling in ultra-deep water, usually described as water depths greater than 5,000 feet, was widely regarded as a vital part of future U.S. oil production. President Barack Obama has said he’ll wait for a 30-day review of the oil spill to decide whether to proceed with new offshore drilling.