Onsen

An onsen (温泉) is a bathing place near a hot spring. Onsen are in Japan. Going to an onsen costs about ¥1000 (US$9), which is about twice as much as going to a sentō bathhouse.
Water.
To count as a real onsen, the water must be at least 25° and have the right amounts of minerals, or dissolved rocks, in it. For example, the water must have iron, calcium and sulfur. Some onsen smell like rotten eggs because of hydrogen sulfide.
Hotels.
Many onsen have a hotel nearby. This kind of hotel is called a ryokan. Not all ryokan have onsen. Some ryokan are 1200 years old.
History.
There have been onsen in Japan for all of history. People have been using the Dogo Onsen on Shikoku for as long as 3000 years.
Manners.
Bathers at an onsen are there to relax, not to get clean. To keep the water from getting dirty, people at an onsen must shower before getting into the water. In very cold weather, it is sometimes all right to get into the onsen just for a short while to warm up, then go shower, then come back. Some onsen allow bathing suits, but most of the time, visitors must wear no clothes at all. This is to keep any clothes that might be dirty out of the water, and keep the water clean. When walking to the onsen, people cover their private parts with a small towel and then put the towel on their heads after they get into the water. People are not supposed to put their heads or hair in the water, and they are not supposed to have tattoos. This is because, in Japan, people think of the Yakuza criminals when they see tattoos.