WASHINGTON – What makes an oil spill really bad? Most of the ingredients for it are now blending in the Gulf of Mexico.

Experts tick off the essentials: A relentless flow of oil from under the sea; a type of crude that mixes easily with water; a resultant gooey mixture that is hard to burn and even harder to clean; water that’s home to vulnerable spawning grounds for new life; and a coastline with difficult-to-scrub marshlands.

Gulf Coast experts have always talked about “the potential for a bad one,” said Wes Tunnell, coastal ecology and oil spill expert at Texas A&M University-Corpus Christi. “And this is the bad one. This is just a biggie that finally happened.”

It’s hard to imagine it not being devastating, said Ed Overton, who heads a federal chemical hazard assessment team for oil spills. The Louisiana State University professor has been testing samples of the spilled crude.

If conditions don’t change quickly, devastation of the highest magnitude is headed for somewhere along the coast, said Overton, who works with the National Oceanic and Atmospheric Administration.

More than 200,000 gallons of oil a day are spewing from the blown-out well at the site of BP’s Deepwater Horizon rig, which exploded April 20 and sank two days later. Crews are using at least six remotely operated vehicles to try to shut off an underwater valve, but so far they’ve been unsuccessful. Meanwhile, high winds and waves are pushing oily water over the booms meant to contain it.

Oil spill experts have drills every few years to practice their response for spills of “national significance.” One took place just last month in Maine. The Gulf of Mexico leak is a “combination of all the bad things happening” and makes it worse than any disaster imagined in the drills, said Nancy Kinner, director of the Coastal Response Research Center at the University of New Hampshire.

Most Americans think of Exxon Valdez when it comes to spills. But the potential and likelihood here “is well beyond that,” said University of Rhode Island ocean engineering professor Malcolm Spaulding. Because the Deepwater Horizon well has not been capped and may flow for months more, it should be compared to a bigger, more dangerous one from a well explosion in 1979, said Tunnell. That was Ixtoc 1, off the coast of Mexico. It was the worst peacetime oil spill on record.

What makes this spill relentless and most similar to Ixtoc 1 is that it’s an active well that keeps flowing.

The type of oil involved is also a major problem. While most of the oil drilled off Louisiana is a lighter crude, this isn’t. It’s a heavier blend because it comes from deep under the ocean surface, Overton said.

This oil also emulsifies well, Overton said. Emulsification is when oil and water mix thoroughly together, like a shampoo, which is mostly water, said Penn State engineering professor Anil Kulkarni. And once it becomes that kind of mix, it no longer evaporates as quickly as regular oil, doesn’t rinse off as easily, can’t be eaten by oil-munching microbes as easily, and doesn’t burn as well, experts said.