By Brendan O'Brien and David Bailey
(Reuters) - Crowds of people are flocking to northwestern Wisconsin to trek on a frozen-over Lake Superior to reach dramatic ice caves accessible on foot for the first time in several years, courtesy of the long frigid winter.
The ice caves on Superior's shoreline are carved out of sandstone by waves from the lake and derive their name from the icy freeze in winter that makes them glisten with hoar frost, icicles and ice formations.
Reachable in warm weather by boat, the caves are accessible in winter only by walking across ice when it is thick and stable enough.
It has been five years since the ice caves were last reachable in the winter, officials said.
About 35,000 people have hiked the more than 1 mile route across the ice in the Apostle Islands National Lakeshore since officials declared the ice a "low risk" on January 15, park spokeswoman Julie Van Stappen said.
"We have never had this number of people coming," she said. "It has been a bit overwhelming, but it has been great for the local community, and they are gorgeous."
A cool autumn and early winter combined with polar vortexes helped lake ice build up weeks earlier than normal, said George Leshkevich, a researcher for the Great Lakes Environmental Research Laboratory.
The Great Lakes in the last week reached its broadest ice coverage in 20 years at 88 percent, with Lake Superior at about 95 percent, according to the research laboratory.
Van Stappen said the round-trip trek to the caves can take three hours or more over a well-packed and slippery path with little cover to break the sometimes fierce winds.
Still, the number of visitors was expected to surge over the three-day Presidents Day weekend, boosting tourist activity in such towns as Bayfield, about 18 miles west of the caves.
Maggie's restaurant in Bayfield was doing more than double the business it normally does in February, which is typically a good winter month anyway because of events such as cross country ski races, office manager Heidi Nelson said.
"Our waiters and bartenders are just elated," Nelson said.
At the Bayfield Inn, manager Tyler Stoklasa said this may be the busiest winter season he has seen.
"We are doing July-type business right now," Stoklasa said.
(This version of the story corrects headline to say "ice caves" instead of "sea caves")
(Reporting by David Bailey in Minneapolis and Brendan O'Brien in Milwaukee; editing by Ellen Wulfhorst and Matthew Lewis)