Most caves have a constant, cool temperature, isolated from the fluctuations on the surface. But in the Mexican town of Naica, there is a cave complex where the air temperature is higher than the highest temperature ever recorded in Death Valley, California! Worse still, the Cave of Crystals (or Cueva de los Cristales) is connected to flooded chambers and must be constantly pumped to keep it drained. This means that humidity is almost 100 per cent; sweating has absolutely no effect. In fact, since the air in your lungs is cooler than the air you breathe in, water tends to condense in them, which leads to progressive respiratory failure. Geologists studying the cave wear respirators and special suits with a network of tubes that circulate water cooled by ice in a backpack. This allows them to work for 20-30 minutes at a time. Without this protection, you would collapse from heat stroke and die within ten or so minutes.