The remote area at the bottom of the Grand Canyon, where searchers are in a fourth day trying to find a missing woman and a teenager, attracts relatively few visitors each year, according to the National Park Service.

Tapeats Creek, where Lou-Ann Merrell and Jackson Standefer lost their footing Saturday during a family trip, is not particularly difficult to hike for experienced backpackers, said Chris Forsyth, president of the Grand Canyon Hikers & Backpackers Association board. But heavy water flowing through the creek can make it challenging, he said.

Merrell is the wife of Randy Merrell, who helped found the Merrell Boot Co.

National Park Service spokeswoman Robin Martin said about 3,500 people got permits in 2015 to camp in the general area where the two hikers went missing, the latest readily available data. About 41,000 total people that year got permits to backpack in the Grand Canyon in total.

The Merrells, Standefer and the boy’s mother were on a path known as Tapeats Trail when the pair fell, authorities said.

The area has a more distinct geology than most of the park and attracts a fair number of visitors but is not as popular with tourists are other spots, like Havasupai Falls. Still, Forsyth, who says he has hiked that area five times, said a visit there calls for a multiple-day backpacking trip and at least some experience in hiking. He said his first trip through the canyon was at Tapeats Creek.

Matthew Nelson, the executive director of the Arizona Trail Association and a former Grand Canyon backpacking guide, said accessing that area on foot takes days and requires an arduous hike. Nelson said the area offers stunning views but is also unforgivingly hot. He said the water in the creek comes from a cave and is always extremely cold.

The Merrell family accessed the area by hiking down from the North Rim.

“The rock that forms the canyon at Tapeats Creek is a particular layer that isn’t found everywhere in the Grand Canyon. It gives it a more unique sense of beauty,” Forsyth said.

An intense search for Merrell and 14-year-old Standefer resumed Tuesday, Martin said.

The search includes three ground teams consisting of about 20 people total, a National Park Service helicopter, a drone and an inflatable motor raft that was flown into the canyon. Search crews are looking within a mile and a mile and half of where the hikers were last seen, as well as where the creek meets the Colorado River.

“We’re really just looking in the water and areas where someone maybe would have been able to get out,” Martin said.

Mark McOmie, the boy’s uncle, said the Merrells are avid hikers and know the area well. He said Lou-Ann and Randy Merrell, who was also on the trip, live in Vernal, a city in eastern Utah. McOmie was not on the trip.

Lou-Ann Merrell is “a very experienced backpacker,” McOmie said. “If they can get to a spot where they cannot be in the water and stay warm, she’s got the skills needed to get them through it. The odds aren’t great.”

The park service said it’s too early to determine what went wrong. No rain or flash flooding was reported in the area, and it was not known whether the water level was higher than usual in Tapeats Creek, a tributary of the Colorado River that runs through the Arizona landmark.

Creeks in the canyon often see higher water levels in the spring as snow melts. Forsyth said that he hasn’t visited Tapeats Creek this year but has been to other parts of the park, where he’s noticed more water than usual, he said.

The McCallie School in Chattanooga, Tennessee, said Jackson is an eighth-grade student at the all-boys school.

A call to Merrell Footlab for comment wasn’t immediately returned.

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Jaques Billeaud reported from Phoenix and Sheila Burke reported from Nashville, Tennessee.