Kugelbunker

The Kugelbunker had a diameter of 2.13m and could sleep four men

A spherical concrete bunker for one person

These bunkers consisted of six segments cast in concrete, the top one of which was different since it included a neckpiece that served as an entrance as well as a fighting position. The other five sections were similar, but side entrances could be chiselled into one or two of them after all the segments were cemented together. 

When the bunker was assembled, its diameter was about 2.1m and its neck was about 37cm high. The interior included a place for four sleeping slabs, although it would not be practical for all four men to stand up at the same time. The man on duty stood on a platform allowing him to occupy the open neck. 

The wall thickness was 4cm, but the interior was designed for metal reinforcements although none was actually used. The entire bunker weighed less than 2 tons, which facilitated its transportation because several specimens could be placed on a trailer, hauled to the site, and rolled into their excavated position. 

https://weaponsandwarfare.com/2017/02/14/the-kugelbunker/