The bathing beauty in the pink suit slid down the four-foot waterfall with a 30-foot scream.

"I'm still alive!" she shouted after reappearing at the surface of the deep, clear pool below the falls. "That was fun."

On summer days, when the sun burns blisters and humidity battles temperature for supremacy, you dream of places to swim like Virginia's Difficult Run. On its steep, frothy fall from Georgetown Pike in Fairfax County to the Potomac River, this creek is all waterfalls and pools, studded with enormous granite boulders and overshadowed by wooded banks.

Huckleberry Finn would build a rope swing here. The Hardy Boys would discover the Secret of Black Pond. And all of them would be subject to arrest by the Park Police.

"Swimming is not allowed there," says Jeri Hall, spokesperson for the National Park Service at Great Falls Park in Virginia, which has jurisdiction over Difficult Run. "But a lot of swimming does go on. It's basically impossible to enforce (the law)."

Park Service officials insist they are not trying to be grinches, stealing this idyllic swimming hole from Washington's sweltering masses. This is not a case of trying to protect a park from people, they say, but of trying to protect people from the park.

"People don't realize how dangerous it is," says one official who has been at Great Falls Park since 1976. "People jump and fall and don't take care of themselves . . . They ignore all common sense. And because it's in such an isolated part of the park, it's very difficult to get them out."

There are signs on both sides of Difficult Run ordering people not to swim, wade or fish below a point near the bridge and parking lot. People swim, wade and fish despite the warning signs. On summer weekends, hundreds of people regularly break the law to beat the heat.

Not all are teen-agers. Hike the trail on either side of the creek and you can see families sharing picnic lunches and kids playing fetch with dogs. Most swimmers, however, are high-school-aged.

"If you go to high school around here, you automatically know about this place," says Bob, 24, who discovered Difficult Run while a student at Herndon High School. "It's just a great swimming hole."

Difficult Run is to Northern Virginia teen-agers what Dickerson Quarry is to their counterparts in Montgomery County. Both are off-limits for swimming and diving. And both take their toll in broken bodies of people who ignore those limits.

Park Police stage periodic raids on swimmers at Difficult Run, making arrests and issuing tickets. For the most part, however, they walk the trail and ask people to leave the water. With an average of 10,000 people using Great Falls Park each weekend of the summer, there are not enough police to adequately patrol all of it. The swimmers know that as well as the Park Police.

"We get out of the water and sit on rocks until (police) leave. Then we go right back in," bragged Bob. As he spoke, a man in his 30s executed a stunning dive into a small pool of water from a cliff 40 feet high.

"You've got to dive straight out. That's the only way to do it," said the diver to some young men who had put down their beer cans to applaud.

An hour later another diver, who looked to be in his early 20s, stepped up to the edge for his own attempt. As soon as he jumped, it was apparent he had not pushed off hard enough. The spectators on the boulders below gasped as the diver twisted his body in a grotesque attempt to avoid an underwater rock he could see approaching. One man watching from the cliff above said, "He's dead!" before the body hit.

Miraculously, the diver escaped with only a fractured arm and a superficial head wound. On Monday he still was in Fairfax Hospital, but listed in good condition.

"I don't know how you keep people from hurting themselves," said a Great Falls Park official. "You can warn them. You can read them statistics. But on a hot day, people are going to go in the river. It's nothing new."