HUDSON, Fla. (AP) – Divers call north and central Florida “cave country” – for the dozens of springs and sinkholes that lead to networks of caverns and twisting tunnels, some hundreds of feet deep and miles long.

The caves can be beautiful. Divers have photographed spellbinding images of sun beaming through aquamarine water at a cave’s entrance. Other photos show divers trekking through towering limestone formations, as if in another world.

But each year, divers get lost in the tunnels. In all, nearly 400 people have died in Florida’s caves, according to the nonprofit International Underwater Cave Rescue and Recovery.

In November, diving buddies Joseph Hartranft and Yessic Spencer plunged into the green water leading to a labyrinth here known as “Wayne’s World” – and a day later, they still hadn’t come out. Soon Pasco County sheriff’s deputies found Hartranft’s 1999 Range Rover parked near the cave network’s sinkhole entrance. Search divers were summoned for a grim task.

What happened at Wayne’s World, and why?

The answers open some of the mysteries of the increasingly popular underground world – the lure of going deeper that many divers feel, the sometimes ad hoc training and regulation for this sport, the community, the science and the unforgiving risks of cave diving.

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The first Florida cave divers began exploring the passages in the 1950s, and the region became one of the world’s most popular cave diving destinations.

Deaths in the caves soon followed. In one tragedy in 1967, four University of Georgia students perished. Even at Wayne’s World, a small wreath marks where a 45-year-old woman drowned.

Despite the deaths, a band of divers – mostly young and middle-aged men – regularly explore the Florida caves.

“We’re always helping each other out. There’s a lot of camaraderie,” said Mike Edmonston, a trainer and dive shop owner who was friendly with Hartranft and Spencer.

Cave diving deaths have been declining since the 1970s, but divers say a new trend is emerging. Before, most of the people who died in caves didn’t have training; these days, more and more do. Technological advances are allowing divers to go deeper, faster before they have acquired enough experience to handle complications that can arise.

Complacency is the primary problem, said Larry Green, training director of the 1,100-member National Association of Cave Divers. “The caves are not killing people. It’s the individual divers not following training guidelines.”

He said he’ll often question divers who have just received their training and quickly move on to advanced dives:

“Does your wife know what you’re doing?”

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“When I’m finished and I’m home, I’ll give you a call,” the 52-year-old Hartranft said in a phone call to his wife and two daughters back in Pennsylvania just before the dive. His son, Joseph Jr., was expecting him home near Tampa by midnight.

When Hartranft and Spencer hadn’t returned the next morning, his son called deputies.

A gate with two metal sheets bearing the words “No Trespassing” prevents cars from pulling in to the Wayne’s World site, which is owned by the National Speleological Society’s Cave Diving Section. Divers must complete 100 safe cave dives after earning their certification before diving the site, among other stringent qualifications.

Hartranft’s son told police his father and Spencer filled their air tanks at a nearby dive shop, Scuba West. In an interview with a detective, an employee said Spencer called the next day asking for the combination to the gate in order to pick up trash. He didn’t mention anything about diving.

With the level of cave diving instruction the men had, they wouldn’t have fulfilled the requirements.

“We’re unaware of them inquiring to dive there at all,” said Jeff Tobey, the scuba shop’s owner. “If they were attempting to get permission from us, they wouldn’t have got it.”

Hartranft had spent 20 years in the Navy and had moved to Florida two years earlier to work for Lockheed Martin. His wife and two youngest daughters stayed in Pennsylvania.

“It was so unlike him to do this,” Robin Hartranft said of her husband’s foray into diving.

Hartranft began taking diving classes about a year ago and, when his daughters, then ages 7 and 9, came down from Dallastown, Pa., to spend the summer, he introduced them to diving as well.

Edmonston said, “I guess he had quite a stressful life…. He wanted something to decompress.”

Spencer was a lieutenant colonel in the Marines who also taught open-water diving. A graduate of the U.S. Naval Academy, he’d been deployed to Kuwait, Afghanistan and Iraq. More recently, he was working at U.S. Central Command in Tampa.

“I was very proud of him,” Yessic Spencer Jr., his father, said.

“He wouldn’t normally do things that were risky, that he didn’t think he could master,” he said of his 42-year-old son, who was married with two young children. “He was level headed.”

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Robin Hartranft said her husband began cave diving a few months before the November day when he and Spencer drove to Wayne’s World.

“I’m a little nervous about it,” she remembers him saying, but he added that he wanted to do “something out of the ordinary that I would never do.”

Divers need to know how to set and follow a guideline so they don’t get lost, and what to do if they get tangled in it. They need to know how to handle themselves when silt, kicked up from a fin or tank, turns clear water cloudy, and what combination of gases to breath at varying depths.

“There’s a lot of math, a lot of physics,” Edmonston said.

Hartranft had diving certification and had begun a class on cavern diving, a preliminary course where divers stay within sight of a cave’s entrance.

Spencer’s certification as an advanced open water instructor meant he could teach divers in lakes and oceans. He’d finished an introductory cave-diving course, the first in a series of classes the industry recommends.

His instructor, Bert Wilcher, said, “We had discussed the fact that he wasn’t fully cave trained at that point, and he was uncommittal as to whether or not he was going to take the rest of the classes.”

In one post in early October on dive site TheDecoStop.com, Spencer expressed frustration at the number of classes required to learn about Trimix, a combination of gases frequently used by cave divers to prevent nitrogen narcosis. The condition induces a drunken-like state of consciousness when too much nitrogen enters the bloodstream.

“I admit that I don’t understand Trimix,” Spencer wrote. “That’s why I’m asking the question. Why do we need three different classes to teach us to dive trimix in the 100-150 ft range?”

“I’m not trying to scrimp on training,” he’d written in an earlier post. “Just curious if we are trying to make trimix training more like PADI (put another dollar in)?” The acronym is actually for the Professional Association of Diving Instructors.

About three weeks before the accident, Robin Hartranft got a worrisome call from her husband. He’d gone diving with someone in Eagle’s Nest, a silty cave about 300 feet deep. Something had gone wrong, and they’d gotten the bends, or decompression sickness.

“Oh my God, Joe,” she told him. “This is not good for you.”

He said his chest hurt and he felt dizzy.

“He sounded horrible,” she recalled. “It was the first time I ever heard him anxious about diving.”

Edmonston said Hartranft didn’t tell him who his partner had been, but one of their tanks failed. To salvage the remaining gas, they had to cut off access to one tank, essentially leaving the diver with half the gas they’d planned.

“I gave him a piece of my mind,” Edmonston said. “I told him, ‘It’s a really bad idea, never do it again. What are you trying to do?'”

When Robin Hartranft spoke with her husband the next day, he told her the incident had been a fluke.

“I’m up here with two kids, and you know, I’m thinking, all right,” she said. “I just thought, something happened and everybody’s OK now.”

Edmonston last spoke with Hartranft about a week before his November dive. Hartranft wanted Edmonston to help him get certification cards to fill up his tanks with more advanced, combination gases that can be used to dive deeper.

Hartranft wanted to dive Wayne’s World and Eagle’s Nest, Edmonston recalled. He refused to give him the cards.

“I said that he had absolutely no business being in Wayne’s World,” Edmonston said. “I told him, ‘Do not go in there.'”

“His reply was, ‘There’s no scuba police.'”

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The Pasco County Sheriff’s Office relies on trained cave divers when something goes wrong inside the limestone passages.

Paul Heinerth and Brett Hemphill know the intricacies of the caves beneath the Wayne’s World sinkhole. After getting the call in November, the two strapped on their gear and went over their protocol; they exchanged “Good luck” and dove to about 40 feet, to where the sinkhole funnels into a narrow passage.

The small opening led to a room, about 45 feet across. Heinerth and Hemphill immediately spotted Spencer. He had one arm wrapped around the guideline leading divers through the cave. A reel of rope was attached to his hip and about 6 feet of line lay on the floor, Hemphill remembers.

“When I saw Yessic, I was just sad,” he said. “He was so close to the entry.”

Neither of Spencer’s fins were on his feet. One was about 20 feet away, and the other just inside a new passageway.

Following the direction of Spencer’s flippers, the men swam about 70 feet through the tunnel before reaching another large room. Hartranft’s body was there.

Hemphill speculates that Hartranft veered off course and soon realized he wasn’t where he was supposed to be. He managed to make it back to the main guideline, but at that point it was probably loose – as the rescue divers had found it. It was also likely difficult to see very far ahead.

Spencer was having trouble with his tanks. He’d resorted to breathing from a bottle of oxygen, which can be toxic at 42 feet and under extreme stress.

From where he secured himself to the guideline, Spencer would have been able to see the passage where Hartranft was expected to emerge, Hemphill said.

“If he had no concern for Joe, he would certainly have grabbed that bottle and crawled out of the cave,” Hemphill said. “He probably turned around or stayed there long enough to pause and look for a dim light, or some sign that Joe was coming.”

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Those who knew the men struggle with mixed emotions.

Robin Hartranft, whose own father died when she was young, said now “my girls don’t have their dad. He loved them. They loved him. And still love him. I’m just angry that he’s not here.”

Together the two men left seven children behind.

“I think the biggest thing is, it made us mad,” Edmonston said. “Because it was completely preventable with available training.”

“But you can’t hold their hands.”