When Sweden invaded Poland in the 17th century, the Swedes made off with pieces of marble lintels, columns, and other architectural details from the Polish royal palace.
Hundreds of years later, Nazis invaded Poland, carrying with them deadly, modern weaponry and a system of violent repression aimed at the country's Jewish population.
Now, thanks to a severe summer drought, evidence of both these invasions is turning up in Warsaw, beached on the dried riverbed of the Vistula.
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.
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.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.