The very oldest saunas were probably only pits dug in a slope in the ground and primarily used as dwellings in winter. There was a fireplace and stones were heated in fire until they were hot. Then water was thrown over them to produce steam and to give a sensation of more heat. This would raise the temperature so high that people could take off their clothes. The word sauna is an ancient Finnish word and its etymology is not clear but it may have originally meant a winter dwelling of this kind.
The earliest form of sauna was used primarily for dwelling and secondarily for bathing. Buildings used as both a sauna and a home were still found in Finland as late as in the 19th century. These were exceptions, though, and the reason for this arrangement was either poverty or temporary use of the building. Other than the very earliest times saunas have always been separate from homes. This was already documented in descriptions from the 12th century.
A modern ground sauna
A more advanced type of the ground sauna had a roof supported by beams and there was a hinged door, even the whole front wall could be made of logs. These kinds of saunas have been in use up to this day and some still consider the ground sauna so good that even new ones are being built.