The Poás Volcano in Costa Rica is home to two crater lakes, and they could not be more different. The first fills an inactive crater, its water is clear blue and its rim lush with vegetation. The other could be accurately described as a hell hole.

Laguna Caliente—literally hot lagoon in Spanish—derives its hellish qualities from the churning of magma underneath the active crater. Sulfur-rich vapor rises out of the lake, gagging anyone unlucky enough to get a whiff and poisoning anyone unlucky enough to get too many whiffs. The water itself is three times as acidic as battery acid. And every once in awhile, a rumbling below shoots a jet of hot, acidic water into the sky. It’s not the kind of place where you want to paddle a boat.

So Guy van Rentergem decided to build a drone boat. Van Rentergem visited Laguna Caliente in 2015 with friends and volcanologists, but he is not a volcanologist  himself. However he is a chemist, a constant tinkerer, and a long-time caver who’s worked with geologists in the past. Next year, he told his colleagues, he’s coming back with an autonomous boat so they can map the bottom of the lake.

“Okay, I made a promise,” he remembers thinking. “So what am I going to use?” What kind of boat can survive several hours in a hot, acidic bath? A plastic one, of course—that miracle of petroleum chemistry. At a flea market near his home in Belgium, van Rentergem picked up a secondhand bait boat. These small remote-controlled boats are usually used to drop fish bait at locations in a lake.

But van Rentergem needed something autonomous; the roiling vapors of Laguna Caliente made it impossible to steer by sight. He stripped the boat down and gave it new brains for autonomous steering. To adapt it to acidic lake water, he took out the propeller, whose metal shaft would have quickly corroded underwater. Instead, the boat got an air propeller that juts up like a mini fan. And most importantly for the purpose of this study, he added a sonar for bathymetry, or measuring the depth of the lake. Van Rentergem didn’t have any acidic lakes nearby in Belgium, so he tested it at a local windsurfing spot. “The owner now has a nice bathymetric analysis,” he says. The boat cost about $700 in materials.

Overview of the crater with the lake at the center (Joyce Lundberg)

Last summer, van Rentergem finally returned to Laguna Caliente with a team including scientists from the Claremont Colleges, Carleton University, and the University of Costa Rica. They donned gas masks and started the 30 minute hike from the rim of the crater. It was hot right next to the lake, and the extra safety equipment didn’t help. It became increasingly desolate as they descended into the fog. “There’s no life, nothing. Just a pure mineral world,” says van Rentergem.

The boat got a firsthand experience of the lake’s harsh conditions. It had taken off just fine, plowing smoothly through Laguna Caliente. But after its second run in the lake, one of those periodic eruptions doused it in acid water. The boat didn’t look good and the wiring short-circuited. But the data survived.

Carlos J. Ramírez (left) and Guy van Rentergem holding the drone boat. (Donald McFarlane)  
The boat on Laguna Caliente (Joyce Lundberg)

The hardest part, it turned out, was getting the boat physically out of the lake. The boat was painted bright yellow, which would have been great if the lake were as blue as the first time the team saw it. But instead, a yellow film of sulfur had formed on the edges of the lake, perfectly camouflaging the boat. Van Rentergem remembers still being up on the volcano, trying to find the boat just a couple hours before his flight left from Costa Rica. Another member of the team, Donald McFarlane, ended up rappelling down a cliff to recover it, and van Rentergem had to carry the boat, still stinking of sulfur, through the airport. “It’s a wonder I got the equipment through customs,” he says.

The data they recovered from the data was odd. “It was like a black hole, and there was no data from the center of the lake,” says van Rentergem. Other teams have reported a possible layer of liquified sulfur at the bottom of the take. Van Rentergem and his coauthors hypothesize that the sulfur that absorbs the sound waves of the sonar, almost like a black hole for sound. The team recently wrote up their results in Eos.

Sulfur floating on the surface of the lake (Joyce Lundberg)

Next year, van Rentergem has new plans for the lake. He’s already rebuilt the boat, and he wants to add equipment for sampling what’s in the lake water, and to test the liquefied-sulfur hypothesis.

But there’s a hitch: There might not be a lake. This April, the volcano under Laguna Caliente erupted, raining rocks and ash down onto nearby buildings. The scene looks totally different from when they first surveyed it. The team might also go to another active crater lake in Costa Rica. Either way, van Rentergem knows one thing for sure: He’s painting the next boat red.