An indigenous Peruvian stands by the famous wall at Cuzco, Peru, built by the ancient Inca, in this 1907 photo from the archives of the U.S. Library of Congress. The two images comprise a stereogram - meaning that if you look at the images and cross your eyes, a 3D version of the photo will appear between the two. For larger versions, visit the LoC page.
Many 'alternative history' authors, including Graham Hancock, have made note of the seemingly impossible size/weight of the blocks, and the ingenious method of cutting and fitting these huge pieces of masonry together so tightly that no mortar is used, and no gaps are left between the stones.
The segment of Inca masonry shown above is not a specialized example - see the image below of the walls of the fortress of Sacsayhuamán to witness the scale of building done in this fashion (click to megasize the megaliths - and I mean that, it's a 20,000px wide image):
(h/t @Jason Colavito)