Maeshowe is an unassuming-looking hill in the middle of the fields in the Orkney Islands. Beneath the peat is a chambered cairn built nearly 5,000 years ago, around 2,800 BCE. The 35-foot entry passage is less than 4 feet in height, so you can imagine the awkwardness of entering for a taller than average guy. The large vertical slabs on either side of the entry are actually not weight-bearing supports, giving rise to speculation that they first existed as freestanding stones, and the rest of the cairn was subsequently built around them.

https://en.m.wikipedia.org/wiki/Maeshowe