Underground city beneath the pyramid of 'Giza'.
A massive subterranean tunnel system with chambers and rooms was discovered below the surface of the Pyramid Plateau. Few known about it today , it was extremely well documented in the past. there was twelve places regularly disposed, interspersed with terrace and arrange around twelve halls. Walls are covered with curved figures. 
https://www.ancient-code.com/there-is-an-incredible-lost-underground-city-beneath-the-pyramids-of-giza/
Built around 12,500 B.C. ......By who is one of the most interesting inquiries of our time.....You may know but Graham Hancock is a good place to start....