Exploring the world’s grossest caves

When Natalie Gibb emerges from one of her cave dives on the southeast coast of Mexico, the 41-year-old woman could easily be mistaken for a swamp monster.

“I’m dripping with microbial goo and reeking of rotten eggs. My skin is burning from the hydrogen sulfide, and I’m grinning ear to ear,” Gibb says. Why the smile? Because she’s just visited somewhere no one has seen before.

“When I was 5, I would say I wanted to be an explorer, and people kept telling me the entire world has already been explored,” she says. “Turns out, that’s not true.”

Since 2014, Gibb and her dive partner, Vincent Rouquette-Cathala, have discovered more than 55 miles of caves beneath mangrove swamps near the border of Mexico and Belize.

Scuba diving in any cave is extremely dangerous and requires years of specialized training, but Gibb’s caves are next-level terrifying – and gross. For one thing, they are often filled with dissolved hydrogen sulfide, a poisonous substance that can be absorbed through the skin and cause headaches, dizziness and nausea.

Plus, it makes you smell like rotten eggs. “People are like, ‘You know, you need to take a shower,’ ” she says. “And I’m like, ‘No, I just took a shower. I’m off-gassing.’ ”

Also gross: Gibb often emerges from dives covered in black slime – the remains of microbes that hitched a ride on Gibb and died the second they touched the oxygen-rich water of the estuary. “For them, oxygen is poison, and hydrogen sulfide is just fine,” she says.

Gibb went on her first dive in 2003 while on vacation in Florida. Four years later, while traveling in Mexico, she was persuaded to go on a guided cavern dive. Caverns are like caves with big open mouths, so you’re never far from the entrance or the surface.

“Before the dive, I thought cave divers were crazy. Afterward, I was like, ‘OK, I am going to be a cave diver. I’m going to have my own cave diving center, and I’m going to train cave divers, and I’m going to explore caves.’ “

Gibb achieved all those goals within a few years, even discovering a small cave system. But her biggest discovery came in 2014 after getting a tip from a fisherman who said there was a hole deep in a mangrove swamp. Gibb and Rouquette-Cathala rented a little boat, found the hole and discovered a cave at the bottom, but the entrance was only about one foot wide – too small to swim though.

Gibb had a hunch there were other caves in the area, so she looked at Google maps and noticed that the water above the hole had a yellowish hue. She searched the map for similar-colored water nearby and systematically began checking them. For years, Gibb kept finding tiny peepholes into what looked like a large, interconnected system of caves. Then in early 2019, she finally found an entrance she could wriggle through – barely.

“I couldn’t see anything, so I was swimming blind. But I felt water flowing toward me, and so I swam against the current. Then I felt water pushing up from underneath me, so I nosedived down into this hole. About 25 feet down, I go through another entrance and find myself in a big chamber completely covered by bright orange sponges.”

Gibb and Rouquette-Cathala had to swim backward to get out of the cave, and Rouquette-Cathala refused to go back in. “I said, ‘Yeah, it’s like really poor visibility, and it’s a little unstable, but I haven’t spent the last five years looking for this place to not dive it,’ ” Gibb recalls.

She did two more dives alone, pushing farther into the cave each time, and eventually found what she was looking for: an anaerobic (without oxygen) ecosystem filled with hydrogen sulfide and surfaces covered in layers of microbes. Gibb collected microbes and sent them to scientists at Northwestern University to analyze. So far, they’ve found DNA from ancient single-celled organisms called archaea.

When she’s not looking for caves, you can find Gibb in her dive shop in Tulum, Mexico. Tulum has cenotes – picturesque, crystal clear caverns that are relatively safe for beginner cave divers. The cenotes are pretty, Gibb says, but she much prefers the mucky, stinky and dangerous caves that she has discovered herself.

“I get to see places that no human on Earth has ever seen before,” she says. “How cool is that?”