A sign warning people not to swim in the river is submerged in Warsaw, Poland, on June 30, 2020. Heavy downpours have caused flash floods in Warsaw on Monday evening, with over 300 cases of flooded cellars, garages, as well as underground stations and tunnels, having been reported. (Photo by Jaap Arriens/Xinhua)

A child is seen playing at a flooded area in Warsaw, Poland, on June 30, 2020. Heavy downpours have caused flash floods in Warsaw on Monday evening, with over 300 cases of flooded cellars, garages, as well as underground stations and tunnels, having been reported. (Photo by Jaap Arriens/Xinhua)

Aerial photo taken on June 30, 2020 shows a flooded beach on the bank of the Vistula river in Warsaw, Poland. Heavy downpours have caused flash floods in Warsaw on Monday evening, with over 300 cases of flooded cellars, garages, as well as underground stations and tunnels, having been reported. (Photo by Jaap Arriens/Xinhua)

Aerial photo taken on June 30, 2020 shows a flooded river in Warsaw, Poland. Heavy downpours have caused flash floods in Warsaw on Monday evening, with over 300 cases of flooded cellars, garages, as well as underground stations and tunnels, having been reported. (Photo by Jaap Arriens/Xinhua)