Near Gigen, Bulgaria, archaeologists have been documenting newly exposed foundations of Constantine’s Bridge, an enormous Roman bridge inaugurated in AD 328 under Emperor Constantine the Great. The bridge stretched for more than 2 kilometres across the Danube, connecting the Roman settlements of Oescus in present-day Bulgaria and Sucidava in present-day Romania. It survived for only a few decades before disappearing from use. Now, nearly 1,700 years later, the river has temporarily revealed parts of its foundations again. And this isn't the only ancient bridge the Danube has given back. In 1858, another severe drop in the river exposed substantial remains of Trajan’s Bridge, built around AD 105 by the Romans during their conquest of Dacia.