Da Vinci's Self-Supporting Bridge

In 1502, Leonardo da Vinci designed a self-supporting bridge for Ottoman Sultan Beyazid II of Istanbul, intended to span the Bosphorus. It consisted only of interlocking beams with no supports underneath, no fasteners, and no mortar. The sultan didn't use the design, because he didn't believe it could possibly work. But it does, because of physics! What's more, the design is simple and portable, making it a useful military tool.

Still, I can't help but think of all the things that could go wrong. Sure, the physics are sound, but it would be built and used by fallible humans. If you used weak wood, it could break. It you used wet or algae-covered wood, a tiny slip by someone walking on it could bring the whole thing down. If your builders make one mistake in construction, it could be a disaster. However, as an engineering concept, it's pretty darn cool. (via the Awesomer)