The Great Pyramids
Some materials engineers did some electron microscopy in order to determine the makeup of the stone used to build the pyramid. The shocking answer? Parts of the pyramids were cast stone. Ancient concrete. This makes the whole process of building the pyramid make a lot more sense. Also explains why they have never found the quarrying tools that would have been needed to carve the blocks out of the ground. I love that the cast blocks have many of the same properties, from a visual appearance level, of actual limestone.
This is too amazing, and makes me feel much better about the future of modern construction. Making portland cement, which is what is mostly used today, is an energy intensive process, producing plenty of pollutants, and generally consumes a fair amount of oil as well. This process, which uses universally found materials and little or no energy, produces excellent limestone style building materials that could be used in many places where concrete is currently used. Very cool. Here is a movie of some french dudes using the process to demonstrate the different types of blocks that could be made: