On a cold, windy, wet summer evening last month, Pete Beachy giggled as he peered into his binoculars and prepared to do something even he admitted can seem very, very stupid. “There she is,” he muttered. “That’s a wave.”

Parked along the Seward Highway in Alaska, Mr. Beachy had spotted the first hint of foam curving around Bird Point, 30 miles southeast of Anchorage, and heading toward an inlet known as Turnagain Arm. The “wave” was a tidal bore, a twice-daily surge of water formed when the incoming high tide from the Pacific Ocean is forced into the narrow arm of Cook Inlet against the outgoing current.

The resulting breaker can reach 10 feet in height, with speeds of 10 to 15 miles an hour — and surfers can ride it for miles if they’re tough enough to withstand the cold and skilled enough to stay upright.

“One moment it’s still, and you’re looking up at the mountains and eagles and goats, and the next you feel like you have the entire ocean coming up behind you,” Mr. Beachy said. “It’s magical, man.”

Hydrologists have documented more than 60 tidal bores worldwide, but only about a dozen are surfable. The Turnagain Arm bore is among the world’s most accessible. The secret is getting out, to big-wave surfers seeking one of the world’s most unusual rides, and to a small but growing number of Alaskans who have embraced the homegrown extreme sport. The tourism agency in Girdwood, a nearby ski town, posts tide charts on its website highlighting the days with the biggest potential waves.

Every newcomer increases the risk, and fears the authorities will decide to shut their fun down.

“Especially with Instagram and all that, it’s kind of become a destination,” said Michelle Weston, fire chief in Girdwood. “But the people coming out here have no idea how little we can do for them if they get in trouble.”

Turnagain Arm got its name because Capt. James Cook’s crew had to turn back repeatedly in 1778 while searching for the Northwest Passage, and reaching the surf now means scaling several stories of steep, jagged riprap — rubble heaped on the shore to protect it from erosion. The water can average 45 degrees, cold enough to rob people of basic motor function within minutes.

The arm is also lined with fine glacial silt that acts like quicksand. Surfers pulled toward the shallows risk getting stuck in dense mud until the tide rises again — the stuff of nightmarish local folklore. Ms. Weston raised money a few years ago to purchase a specialized mud rescue device for her department, but that requires three people and the ability to reach stranded surfers in time.

In June, a 25-year-old man from Anchorage who drove down to paddleboard the bore tide died after getting trapped by the current and mud and becoming too exhausted and cold to reach shore. Rescuers carried an inflatable boat to the water, but a three-foot chop filled it with water, injuring a firefighter, and making it impossible for the motor to overcome the current. No Coast Guard helicopters were available.

“This wasn’t even some sort of wild water drowning,” said Karolina Zakravska, a public information officer with the Alaska Department of Transportation. “He was just stuck out there because the current was too strong for him to get back.”

Mr. Beachy also watched the young man struggle, but rescuers talked him out of paddling out to help, something he said he has done “more times than I want to remember.” Three days later he tried to dissuade another newcomer, wearing “a dry suit with Vans skate shoes, socks on his hands, a large climbing backpack and a kayak paddle.”

“Some of these kids don’t get that in Alaska, you need a Plan B, a Plan C and a Plan D,” he said, “because no one is coming to rescue you.”

Mr. Beachy first paddled into Turnagain Arm in 2010 and has been surfing the bore tide as long as anyone still riding. From late spring through early fall, he surfs nearly every day and offers paid guide services. He used to surf well into October, dodging shoreline ice and slush, but now stops in September to go moose hunting.

His winters are spent on Oahu’s North Shore in Hawaii, where he says he learned how to steer unprepared newcomers away from the big waves. He’s afraid of more injuries and that the recent death will prompt officials to crack down on surfing the bore tide. Ms. Weston said Girdwood Fire & Rescue already receives several calls a week in the summer from passers-by worried about people on the mud flats.

On sunny days, 20 or even 40 surfers can line the arm waiting for a ride, and the Seward Highway pullouts can feel like a slice of Southern California rather than subarctic Alaska. During the recent cold summer rain, the only surfers beyond Mr. Beachy were a truckload of high school friends down from Anchorage and a newcomer from California.

“Sometimes this is better than the big waves,” Mr. Beachy said as he waited for the evening tide.

The bore begins near Anchorage and takes two to three hours to reach him. Once he spots it through his binoculars, Mr. Beachy has 15 minutes to pull on his wet suit and gloves, lift his board from his weather-beaten white van and scramble down the rocks. Then he waits, doing breathing exercises and staring at the Kenai Mountains on one side and the peaks of the Chugach range on the other.

The wave appears as a ripple on the horizon, but by the time it reaches surfers, the water’s roar drowns out the highway traffic. Catching the wave can mean the ride of a lifetime. Mr. Beachy once cruised the bore tide for six miles, nearly an hour, according to his GoPro.

“It’s a crazy thing to do, I know,” he said. “But you can’t ride an hour in Hawaii.”