In concrete jungles, there's the fear of falling through a sidewalk grate. In parts of Hawaii, there are lava tubes.

They form during volcanic eruptions, starting their lives as rivers of lava and flowing downstream as they branch out into smaller channels like the roots of a tree. Then the exposed lava cools and hardens. A roof forms over the river, allowing the lava beneath it to go on flowing for months until it slows and drains and hollows - becoming a tube.

Despite their ubiquity on Hawaii's Big Island, it's rare for someone to actually fall into a lava tube, experts have said. But it can happen.

And on Monday, police said it happened to an elderly man - in his own backyard.

The man, reportedly in his early 70s, appeared to be trimming branches in his yard this week when he fell "through a soft area of ground" into a hidden lava tube on his property and died, according to a Wednesday statement from police on Hawaii's Big Island.

Police arrived at the man's home in Hilo, on the island's east coast, to perform a welfare check Monday after one of the man's friends called to report him missing, Big Island Now reported. Rescue personnel discovered him resting at the bottom of the two-foot-wide lava tube, 22 feet below ground.

He was transported to the hospital, where he was pronounced dead, police said. An autopsy performed Tuesday found that he died "as a result of injuries consistent with falling," and no foul play is suspected, police said.

It's unclear exactly how the man fell into the lava tube. Ken Hon, a volcanologist at the University of Hawaii at Hilo, told The Washington Post he suspects that the hole was already there and that the man likely just didn't see it ― possibly because it was covered in overgrowth. Plenty on the island are, Hon said. The tubes are everywhere: in neighborhoods and underneath streets, in forests and national parks like an underground cave system right beneath your feet.

"You could be standing on one and not even know it," Hon said.

The holes, he said, are called "skylights." "That's where a thin part of the roof collapses, so you'll have a hole into the lava tube," he said. "If you think of a skylight in a house, it's just a window up in the roof. The skylight is a hole in the roof of the cave."

Hon said he couldn't be certain which volcanic eruption created the lava tube in the man's backyard, or even whether it was definitely a tube. But Hon and two other scientists who spoke with The Post believe there's a good possibility that the lava tube in question formed during the massive 1880-1881 eruption of the Mauna Loa volcano. The lava flowed for months and miles, threatening the town of Hilo as it slowly inched closer. People prayed to God and Pele, the Hawaiian goddess of volcanoes, asking for the lava stop, building ditches and blasting dynamite to try to divert its flow, according to the National Park Service.

More than nine months later, once the rivers of lava eventually stopped, the result was the Kaumana Caves - a 25-mile network of lava tubes.

Tom Shea, a volcanology professor at the University of Hawaii at Manoa, said tube systems are typically structured "in a treelike fashion." The primary tube is the trunk, sometimes stretching 30 or more feet in diameter, while secondary tubes are like branches, growing smaller with distance, he said.

"The difficulty is therefore that even if the main tube location is known, there are still hazards associated with secondary tube branches," Shea wrote in an email to The Post.

The man who died, Shea said, likely fell into "the prolongation of the well known Kaumana Caves on the west side of Hilo."

Shea said the lava tubes are a "major issue for land stability" on the Big Island, and that geological surveys typically detect them before construction of buildings and homes. The roofs of the tubes may "support significant weight for years, decades, or more and never collapse," he said.

Or they can weaken over time, he said, weathering or fracturing beneath weight - and collapse.

While it remains unclear how often people have been victims of the lava tubes, media coverage over the decades has offered a window into the seriousness of these incidents.

In 1941, the event appears to have been rare enough that a Hawaii short story contest winner wrote: "Accidents have occurred, sometime deaths, in falling into lava tubes. Until one has experienced the above, he may either believe or may say, it is all superstition."

But many have lived to tell the horrifying tale.

Art Carter, a famous volcano photographer of the '50s and '60s, was apparently so unlucky that he fell into a lava tube three times during his career - including one occasion in which he collapsed 12 feet through a freshly formed, too-thin crust of lava not long after a Puna volcano eruption.

The same would even happen to government scientist Ed Wolfe about 30 years later - only that in his case, the tube was still steaming hot.

"What has to be every geologist's nightmare - falling into a red hot lava tube - happened last Saturday to Ed Wolfe, staff geologist with the Hawaiian Volcano Observatory," began the 1983 Hawaii Tribune-Herald article alarmingly headlined, "Scientist falls into hot lava tube."

"The lava tube was dry but it was still incandescent," Wolfe told the paper, after suffering second-degree burns. "I rode the piece of the roof down to the bottom of the tube. It was about waist deep on me. You can't believe how hot it was!"

But in cases more similar to the septuagenarian's, other people have simply fallen through a hole they couldn't see - sometimes while doing routine activities.

In 2002, a firefighter survived a fall of more than 100 feet into a lava tube in Hawaii's Fern Forest while searching for three lost pig hunters in the forest, according to the Honolulu Star-Bulletin. The hole was about five to eight feet wide, obscured by a fern, the paper reported.

"My friend said one second I was there, and then one second I was tumbling down," the 41-year-old firefighter told the newspaper.

In 2011, a 59-year-old woman fell 15 feet into a lava tube while hiking on a trail within the Kaumana Caves system. And that same year, a 63-year-old man fell 40 feet down a lava tube while surveying a vacant lot he had just purchased in Pahoa.

Deaths appear to be significantly more uncommon, Hon said, adding that he was not immediately aware of any incidents comparable to the fatal case this week. A Hawaii County Civil Defense Agency administrator told the Honolulu Advertiser in 1997 that one case came to mind: In the mid-80s, a Japanese tourist took a fatal fall into a lava tube during an ill-advised night hike.

Hawaii residents don't often worry about falling into lava tubes, Hon said. Most lava-tube holes are visible from the surface, "so you tend not to walk into the hole - it's like an open manhole cover."

But when Hon takes people to see active lava flows, in search of a skylight into which visitors can peer to find a roaring river of lava, "you watch people start to think about it," he said.

"There's a hole in the ground, and you see this liquid stream of lava - red hot lava - going underneath you. And you just watch them think," he said. "They look back over what they've walked on, and they realize, how many of these did I walk over, and not have any idea they were there?"

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