The cave has to be the oldest form of shelter in human history, and from a scientific perspective it’s actually a pretty good dwelling!
Caves are either naturally occurring or man-made. Given their temperature is regulated by the planet, caves tend to stay warm in the winter and cool in the summer. They save up to 80% on climate control costs. Their materials (rocks, stones & dirt) are surprisingly good with sound insulation, and many caves have underground water sources. Also, caves with only one entrance offer enhanced security. On the negative side, it’s a cave. Typically what you see is what you get, and unless you have a second exit, you’re pretty much doomed if there’s a cave-in or intruder entering.
(Thanks to Kim Nguyen for suggesting this time-tested yet oft-overlooked dwelling.)