The monumental reservoirs are one of Łódź's most mysterious places, hidden underground on Stoki Hill, state news agency PAP reported.
Built in the 1930s, they still form part of the city's water-supply infrastructure and are normally filled with drinking water, and are therefore closed to visitors.
Their interior resembles a Gothic underground cathedral, with 81 columns, 100 domes and more than 200 brick arches.
The reservoirs measure 60 by 60 metres and were designed to withstand the pressure of 60,000 tonnes of water.
They were designed by English engineer William Heerlein Lindley as part of Łódź's modern water-supply system.
“Parts of the floor are concave, while the walls are also curved, so that the weight of the 15,000 tonnes of water is distributed evenly throughout the reservoir. The bricks used in construction were also specially fired, making them waterproof and able to withstand enormous pressure,” said Miłosz Wika, spokesperson for the Łódź Water and Sewerage Company (ZWiK).
Their elevated location allows water to flow by gravity to lower parts of the city. The reservoirs receive water from 19 deep wells.
The Łódź reservoirs are opened to visitors only when they are temporarily emptied for inspections and technical work.
This year's opening period runs until end of month, with more than 6,000 people expected to visit. Tickets sold out in one day.
Visits last up to 60 minutes. The temperature inside is around 10°C, so visitors need warm clothing and non-slip shoes.
(sp)
Source: PAP