A few minutes in this hot spring would leave you feeling refreshed. But what would happen if you camped out in this foul-smelling pool for 24 hours? How could this water improve your health? What could happen to your brain? And how soon would your organs get cooked?

Humans have used hot springs for thousands of years to relax and treat medical conditions . These waters shoot up from below the Earth’s surface, heated by churning magma. And the water brings sulfuric compounds that create that fragrant aroma. All that ancient hot rock below boils these waters to temperatures around 50 °C (122 °F). I know it smells, but it feels great.

In Japan, authorities recommend you only stay in some hot springs for 40 minutes at the most. But what would happen if you fell asleep in this hot pot? This hot spring looks so warm and inviting. Why could it be so dangerous? Well, what lies beneath the waters could be fatal. But we’ll get back to that.

While these waters can feel soothing, the temperature might reach near 100 °C (212 °F) if this spring becomes superheated. If it got to just the right temperature, this highly acidic water could burn your skin within 10 minutes. So a whole day might cook you like a hard-boiled egg. But if this water stayed at medium heat, could soaking in it replace your workout?

A 2015 study in Finland found that one hour in a hot spring could burn off as many calories as a 30-minute walk. Unfortunately, the sample group only included men. OK. Don’t get too excited. Your body would be working overtime to deal with the excess heat. That means you’d be using more energy to stay at a normal temperature.

As the hot springs simmer you, the real test of your body’s limits would begin. You need to maintain an internal temperature of 37 °C (98.6 °F) to keep your body systems working. But the danger starts if your temperature rises just a few degrees. You could get hyperthermia, a dangerous condition that could have consequences long after you dry off.

You could experience heatstroke or even heat edema, which would make your hands and feet swell. You may not survive unless you get out and into a cold bath right away. And even if you cool down, there’s a chance that you could experience organ and neurological dysfunction within a year. During this bath, your heart rate would increase, and you would start feeling dizzy.

Try to stay awake because you might faint and slip under the water. And keep your nose plugged because deadly microbes might lurk beneath this hot water. While most living organisms may not stand a chance in this heat, the Naegleria fowleri amoeba thrives in hot springs. This microscopic threat could travel up your nose to your brain. And within a week after you feel symptoms like fever, nausea and a stiff neck, you could be dead.

But while you’re in the hot spring, you’d keep sweating and get dehydrated. You’d lose body fluids at about 1.5 L (50 oz) or more per hour. As the electrolytes and minerals left your body, your arms, legs and abs could start cramping, sending you into a world of hurt. When you lose control of your muscles, this hot spring could become a giant toilet. OK. Seriously, let me out now.

Now it’s just a matter of time before your blood vessels leak. This could cause violent hemorrhaging and make you puke blood. And your intestines would release toxins into your bloodstream. Your organs would swell up like steamed sausages, and that includes your brain. If you managed to crawl out of this cesspool, your body would be a wreck. And that’s if things go well.

After the 1995 heatwave in Chicago, 28% of heatstroke survivors died within a year from complications. And 51% were left with permanent organ dysfunction and brain damage. The next time you visit one of these natural wonders, enjoy soaking. But remember to stay hydrated, watch your time and keep your head above water.

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