Under-bed fireplace - GIF

[ranald]

It's known as a "kang" and they date from as early as the first century. Lots more here...

https://en.wikipedia.org/wiki/Kang_bed-stove

I've heard they even name babies due to the ancient traditions: spose a bit like Jackson being the son of Jack but with more suggestive meaning.

[Loose Ctrl]

Those fire places are called rocket mass heaters or rocket mass stoves. The guy that own the permies website, Paul, has a bunch of videos out about building them and how to make them very efficient.

https://permies.com/forums