RILEY TOWNSHIP – Fez Abramson was hiking north on the Appalachian Trail when the route seemed to hit a roadblock.

Ahead lay a cluster of jagged, cabin-sized boulders. To the left and right, the steep, rocky walls of two mountains hemmed in the trail. Then something caught her eye: a white arrow painted at the bottom of one of the boulders. The arrow pointed down at a dark gap in the rocks, less than 30 inches wide.

“I thought, ‘What is this? This is the trail? I have to go through there? This is pure unadulterated madness!”‘ said Abramson, of Bethel.

Abramson, an instructor with the Outward Bound School, was recalling her first hike through Mahoosuc Notch, the notorious section of the Appalachian Trail that is considered by many to be the most challenging mile on the nearly 2,170-mile footpath from Georgia to Maine.

Mahoosuc Notch is a narrow gorge between Fulling Mill and Mahoosuc mountains. The trail travels through a boulder-filled obstacle course that hikers scramble over, squeeze between and slide down. In several places the jumble of boulders creates caves where hikers must crawl on hands and knees, removing backpacks to squeeze through narrow passageways.

The rocks are slippery and often covered with moss. Cool, dank and lush, the place has the feel of a temperate rain forest.

The Appalachian Trail enters Maine from New Hampshire, traversing a heart-pounding series of ascents and descents of 4,000-foot mountains in a region known as the Mahoosuc Range. The Maine Appalachian Trail Club publishes a relief map that shows the range in profile. The spiky up-and-down route resembles an electrocardiogram.

Mahoosuc Notch lies in the heart of the range. Despite its difficulty, and perhaps because of it, the Notch is a popular hike. Warren Doyle, 54, of Hays, N.C., recently made his 13th hike through Mahoosuc Notch.

“It’s the slowest mile on the A.T.,” said Doyle, resting briefly during a rainy day-hike through the Notch in July.

Doyle is a hiker who knows about covering ground. In 1973 he made the first of several thru-hikes of the entire Appalachian Trail. He completed that trip in just 661/2 days, averaging more than 30 miles a day.

“It’s the risk that makes Mahoosuc Notch slow. You use a lot of your arms and legs going through the caves. If you go too fast you could really hurt yourself. As difficult as it is, when the rocks are wet it makes it a lot slicker,” he said.

Doyle has a fantasy about the Notch.

“I always thought this would be a good place for a TV reality series, like ‘Survivor,”‘ he said. “You could have teams going through, trying to pass each other. Mike everybody, have cameras mounted on the cliff walls, trying to see teams going through the small holes at the same time.”

“It’s a big playground for adults,” said Charles Dona, who was hiking with Doyle.

Somehow trees and other plants manage to grow among the boulders. In many places the trail is so shaded that ice lingers well into July. Cool, misty vapors can be seen rising on hot days from the caverns where the temperature may be 15 degrees cooler than on top of the rocks.

“Its like air conditioning,” said Kathleen Moore, 25, of Schenectady, N.Y., who was hiking the Mahoosuc Range with her father.

A few years ago, Moore passed through Mahoosuc Notch on a monthlong southbound hike with some friends.

“We hiked 350 miles, from Baxter State Park (the trail’s northern terminus) straight through to Crawford Notch in New Hampshire,” said Moore. “This was the best mile. I love it. I don’t think there’s anything else like it. You get sick of seeing the same birds, the same trees, the same views from every mountaintop.”

Justin Walker, a southbound hiker walking from Maine to his home in Locust Grove, Ga., paused when he entered the notch to photograph some of the boulders. The Notch was a paradise to him.

“I do a lot of rock-climbing in Georgia. This is like, man, if I had this in my backyard I’d be going crazy,” he said, looking around at all the possible climbs. “Maybe I’ll just stay here and play for a while.”

Carrie Tweeten, 43, and her two sons, Kent, 14, and A.J., 10, of Crystal Lake, Ill., are section-hikers, spending a month or so each summer until they complete the entire trail. This year they are hiking Maine’s 281 miles of the Appalachian Trail. They started in western Maine near the New Hampshire border and spent the first night at the Full Goose shelter, a couple miles south of Mahoosuc Notch.

Their trip through the Notch took the family more than three hours to complete.

“It was raining when we started. Every rock was like a death wish,” said Carrie Tweeten. “The caretaker at the shelter told us it was like a Jungle Gym down here. Well, it’s like the Jungle Gym you were scared to death to go on as a kid.”

Still, she was proud she and her boys made it through the notch in one piece.

“My legs are rubbery, like they feel at the end of a day of skiing,” said Tweeten.

Just north of the Notch the trail climbs Mahoosuc Arm, one of the steepest two-mile climbs on the entire trail.

“I figured we’d tackle the hardest part of the trail while I was still young,” she said. “We’ll save those easy states like Maryland and Virginia for when I’m in my 60s.”