honorthegods:

Roman bridge, Mérida, Spain. Photograph by Ángel M. Felicísimo, 2001 via Wikimedia Commons: X. License: CC BY-SA 2.0

This bridge across Guadiana River was built in the 1st century CE for the Roman colony of Emérita Augusta, now Mérida, a UNESCO World Heritage Site. At 755m (about half a mile) in length, it is the longest surviving Roman bridge in the world, and is still open to foot traffic (guide here).