Coral Castle ! I was there once with the girlfriend. Not even the heaviest unexplained blocks of stone on earth....
Beginning in 1920, Edward Leedskalnin, a 5-ft. tall, 100-lb. Latvian immigrant, began to build a remarkable structure in Homestead, Florida. Over a 20-year period, Leedskalnin single-handedly build a home he originally called "Rock Gate Park," but has since been named Coral Castle. Working in secret - often at night - Leedskalnin was somehow able to quarry, fashion, transport and constructed the impressive edifices and sculptures of his unique home from large blocks of heavy coral rock.
It’s estimated that 1,000 tons of coral rock were used in construction of the walls and towers, and an additional 100 tons of it were carved into furniture and art objects:
An obelisk he raised weighs 28 tons. The wall surrounding Coral Castle stands 8 ft. tall and consists of large blocks each weighing several tons. Large stone crescents are perched atop 20-ft.-high walls. A 9-ton swinging gate that moves at the touch of a finger guards the eastern wall. The largest rock on the property weighs an estimated 35 tons. Some stones are twice the weight of the largest blocks in the Great Pyramid at Giza. All this he did alone and without heavy machinery. No one was ever witness to how Leedskalnin was able to move and lift such enormous objects, although it is claimed that some spying teenagers saw him "float coral blocks through the air like hydrogen balloons."
Leedskalnin was highly secretive about his methods, saying only at one point, "I have discovered the secrets of the pyramids. I have found out how the Egyptians and the ancient builders in Peru, Yucatan and Asia, with only primitive tools, raised and set in place blocks of stone weighing many tons."
The Temple of Jupiter at Baalbek, Lebanon has a foundation that contains the three largest stone blocks ever used in a man-made structure. Each block is estimated to weigh as much as 1,000 tons! No super crane in existence today could lift one, yet they are positioned together with such precision that not even a needle could fit between them. Nearby is an even bigger stone. Known as Hajar el Hibla - the Stone of the Pregnant Woman - it lies abandoned in its quarry, never used. But the giant rectangular block is the largest piece of stone ever cut by humans, weighing an incredible 1,200 tons. It is estimated that it would require the strength of 16,000 men to even budge it, and represents a formidable challenge to 20th century machines and technology.