Bathhouses play an essential role in Japanese culture and are used extensively to wash, relax and socialize. No mere bathhouse can call itself an onsen (hot spring), however. Only baths containing water from a natural spring that gushes or is pumped at more than 25°C can lay claim to this title.
Unsurprisingly then, onsen in Tokyo are a bit a thin on the ground. Instead you’ll find plenty of sento (public baths), a cheaper city alternative to an onsen that uses tap water in its baths.
Photo courtesy of Ken Lee via Flickr.