A huge cargo of elaborate marble stonework that sank to the bottom of Poland's Vistula River four centuries ago has reappeared after record-low water levels revealed the masonry lying in the mud on the river bed.
Archeologists 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. The booty likely sank to the bottom when the vessels capsized.
Researchers knew about the artifacts – on the river bed where the Vistula passes through the Polish capital – but, before the drought, several metres of water stood in the way of retrieving them.
The masonry – large blocks of carved marble which were used in the columns, fountains and staircases of Polish palaces – is now lying exposed, apart from a coating of foul-smelling yellow mud.
"The drought helped us a lot because what had been lying underneath is now at the surface," said Hubert Kowalski, deputy director of the University of Warsaw Museum, who is leading the effort to retrieve the marble stonework.
Speaking at a building owned by the Warsaw river police, where some of the stonework is temporarily being stored, he said historians' knowledge about what happened four centuries ago had previously been sketchy.
"Now we have evidence, the best material evidence of the Swedish invasion so far."
Low rainfall over the past few months has brought the Vistula, Poland's longest river, to its lowest level since regular records began 200 years ago.
Navigation along the river has been affected and officials say if water levels do not soon recover, power stations in Warsaw that use river water for cooling may be forced to close down.
The receding water has also revealed relics from Warsaw's bloody history during the Second World War. During that period, the city was occupied by Nazi Germany and the Jewish population was wiped out. The city rose up against the occupation before the Soviet Red Army arrived and imposed its own rule.
Unexploded Second World War ordnance was found on the river bed in one part of the city over the weekend. Mr. Kowalski said a few pieces of Jewish matzevah, or gravestones, had been discovered on the stretch of river bed he had been studying.
He said they will be handed over to the city's Jewish Historical Institute. Finds of Jewish artifacts are quite common in Warsaw – the legacy of successive Nazi and Soviet schemes to demolish traces of the city's Jewish community.
Historians believed that the Swedes who invaded Poland in the 17th century planned to move the looted cargo up the Vistula to Gdansk, where the river joins the Baltic Sea, and transport it home from there. There is still no firm explanation as to why the boats sank on their journey.
Mr. Kowalski said he and his team have so far located up to 10 tons of stonework, but this is only the beginning.
"The boats had a capacity of 50-60 tons [each], so we think that we should find much more," he said.
Once it has been removed from the river bed and catalogued, the plan is to take the masonry to Warsaw's Royal Castle – one of the sites from which, historians believe, it was stolen by the Swedish invaders.
For now though, 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 researchers to bring in floating cranes.
"We need to wait until it gets higher," Mr. Kowalski said.