A huge cargo of elaborate marble stonework that sank to the bottom of Poland’s Vistula River four centuries ago has re-appeared after a drought and record-low water levels revealed the masonry lying in the mud on the river bed. Archaeologists believe the stonework was part of a trove which 17th-century Swedish invaders looted from Poland’s rulers and loaded onto barges to transport home, only for the booty to go to the bottom when the vessels sank. Researchers knew about the artefacts, on the river bed where the Vistula passes through the Polish capital but, before the drought, retrieving them was a painstaking task because they were under several feet of water. Navigation along the river has already been affected and officials say if water levels do not recover soon, power stations in Warsaw that use river water for cooling may be forced to close down. The low water levels that revealed the artifacts are hampering efforts to retrieve them. Regular lifting equipment would sink into the mud, but the river is too low for the researchers to bring in floating cranes.