At 290 feet long, 160 feet wide, the Temple of Jupiter Baal ("Heliopolitan Zeus") in Baalbek, Lebanon, was created to be the largest religious complex in the Roman empire. As impressive as this is, one of the most impressive aspects of this site is almost hidden from view: beneath and behind the ruined remains of the temple itself are three massive stone block called the trilithon.
These three stone blocks are the largest building blocks ever used by any human beings anywhere in the world. Each one is 70 feet long, 14 feet high, 10 feet thick, and weigh around 800 tons. This is larger than the incredible columns created for the Temple of Jupiter, which are also 70 feet tall but measure a mere 7 feet -- and they weren't constructed from single pieces of stone. In each of the above two images, you can see people standing by the trilithon to provide reference for how large they are: in the top image a person is standing to the far left and in the bottom image a person is sitting on a stone about in the middle.
Beneath the trilithon are another six huge building blocks, each 35 feet long and thus also larger than most building blocks used by humans anywhere else. No one knows how these stone blocks were cut, transported from the nearby quarry, and fit so precisely together. Some are so amazed at this feat of engineering that they have created fanciful tales of the Romans using magic or that the site was created centuries earlier by an unidentified people who had access to alien technology.
The fact that people today are unable to imagine how the construction was accomplish is not license to make up fairy tales, though. There are so many things which we today can do which the ancients couldn't even imagine; we shouldn't begrudge them the possibility that they could do a thing or two which we can't figure out yet.