Sweat.

The sweat lodge (also called purification ceremony, sweat house, medicine lodge, medicine house, or simply sweat) is a ceremonial sauna and is an important event in some North American First Nations or Native American cultures. There are several styles of sweat lodges that include a domed or oblong hut similar to a wickiup, or even a simple hole dug into the ground and covered with planks or tree trunks. Stones are typically heated in an exterior fire^[1] and then placed in a central pit in the ground. (from Wikipedia)

My best friend, Josh, built a sweat lodge for his birthday and invited Chris and I over to participate. It was a beautiful, crisp night. In total, we were probably in the hut for an hour, getting out every 20 minutes or so to give ourselves a break from the heat. In the description above it doesn't mention that you pour water over the hot rocks in order to create a lot of steam. It gets incredibly hot in there and is extremely purifying. Afterwards we lounged around and ate a healthy meal. It was a wonderful evening. Thank you Josh!

Josh built the frame of the hut out of willow branches.

There were 12 river rocks in the fire getting nice and hot.

Josh put mats down for us to sit on.

You cover the frame with tons of blankets which keeps the heat and steam in.