Asked by Reddit to name the creepiest thing he’s ever dealt with in space, former astronaut and retired US Navy captain Scott Kelly didn’t hold back. In the video, posted Wednesday, he revealed it was a huge floating orb of sulfuric acid and astronaut pee.

“I had to clean up a gallon-sized ball of urine mixed with acid,” Kelly told Biography.com in a video interview produced with Reddit. “Essentially, the sulfuric acid that is mixed with the urine basically keeps the toilet from clogging up.”

“If you could imagine, something like Drano.”

The specific piece of hardware that Kelly’s referring to here is the International Space Station’s E-K pre-treatment toilet tank. The E-K introduces astronaut urine to a mixture of water, sulfuric acid, and chromium oxide — the latter of which helps to prevent oxidation and gives the solution an alien-like (but also, maybe, less gross) purple color. The top photo on this article doesn’t actually depict the urine-acid ball but approximates what it might look like; the green orb is actually an experiment Kelly once ran on the ISS using water, green food coloring and an antacid tablet designed to visualize water’s complex physics in zero gravity.

Nearly all varieties of stainless steel contain at least 10.5 percent chromium, and it is the invisible surface layer of chromium oxide created by these oxidation reactions that helps to keep that steel from rusting (that is, oxidizing). You can imagine how important adding more of that chemical might be for restroom facilities inside of a hermetically sealed can of human beings orbiting the Earth.

Astronaut Akihiko Hoshide with a space toilet on the ISS. Wikimedia

While you’re here thinking about it: the ISS had a major overhaul to its bathroom system in 2008, installing a system that actually harvests all wastewater and recycles it into potable drinking water. This wastewater includes: all of the astronauts’ urine, the water used to brush their teeth and conduct other personal hygiene activities, and even the excess moisture in the air.

During the so-called Year-Long Mission (Ron Howard voice: “It was 11 months.”), NASA estimates that Kelly consumed somewhere around 193 gallons (730 liters) of water recycled from the team’s urine and sweat (as well as his own).

Kelly is, of course, only one in a long line of space plumbers. This lucrative, high-demand trade job has also been performed by Japan Aerospace Exploration Agency astronaut Aki Hoshide; cosmonaut Oleg Kononenko; and Italian European Space Agency astronaut and Italian Air Force pilot, Samantha Cristoforetti.

While the urine-acid orb might be the creepiest or grossest thing that Kelly has ever experienced in space, it’s very likely not the nastiest thing anyone has ever dealt with up there. That distinction would have to go one of the innumerable horror stories to come out of the Soviet space station Mir — which was plagued by hidden orbs of watery mold, fires, porthole-damaging fungal infections, and the occasional power failure. (Mir was like an RV driven by a touring 80s hardcore band. It was gross and it ruled.)

If you think you can handle it, you can watch the rest of Kelly’s admittedly less scatological AMA answers here.