By Dagmara Leszkowicz

WARSAW (Reuters) - 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.

Now though, the masonry - large blocks of carved marble which were used in the columns, fountains, and staircases of Polish palaces - is 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, leading the effort to retrieve the marble stonework.

Speaking at a building owned by the Warsaw river police, where some of the stonework is being temporarily 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."

JEWISH ARTEFACTS

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 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 receding water has also revealed relics from Warsaw's bloody history during World War Two. During that period the city was occupied by Nazi Germany, the Jewish population was wiped out, the city rose up against the occupation, and then the Soviet Red Army arrived and imposed its own rule.

Unexploded World War Two ordnance was found on the river bed in one part of the city at the weekend. Kowalski said on the stretch of river bed he had been studying, a few pieces of Jewish matzevah, or gravestones, had been discovered.

He said they would be handed over to the city's Jewish Historical Institute. Finds of Jewish artefacts 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 from there transport it home. There is still no firm explanation of why the boats sank on the way.

Kowalski said he and his team had so far located up to 10 tonnes of stonework, but this was only the beginning. "The boats had a capacity of 50-60 tonnes (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 looted by the Swedish invaders.

For now though, the low water levels that revealed the artefacts 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.

"We need to wait until it gets higher," Kowalski said.

(Reporting by Dagmara Leszkowicz; Editing by Christian Lowe and Robin Pomeroy)