The huge new structure, buried 30 metres underground next to a railway station, is a key part of efforts to clean up the River Seine, which will host swimming events during the Paris Games in July and August. "It's a real cathedral. It's something extraordinary," said Paris Mayor Anne Hidalgo on Thursday as she walked along the bottom of the huge cylindrical structure, which took more than three years to complete.

Paris deputy mayor Antoine Guillou has compared the project in western Paris, near the Austerlitz transport hub, to Notre Dame, which will be rebuilt in 2019 after a devastating fire. "I like to say that we're building two cathedrals. There's the one above ground that everyone knows, Notre Dame. And then there's the underground one," he told reporters during a visit in mid-March.

Notre Dame will not be ready in time for the Paris Games, as promised by President Emmanuel Macron promised in the immediate aftermath of the inferno that tore through the 850-year-old masterpiece, but its spire has been restored and workers are busy on the roof ahead of its grand reopening in December.

Fortunately for Olympic open-water swimmers, the rainwater facility is due to go into operation in June, following tests later this month. Its role will be to store rainwater in the event of a heavy downpours, reducing the chances of the capital's sewage system having to discharge its pathogen-laden contents directly into the Seine.

Paris' sewage system is under intense scrutiny after Olympic organisers pledged to use the Seine for marathon swimming and triathlon events during the Games, which begin on 26 July.

Cleaning up the river has also been promoted as a key legacy of Paris 2024, with Hidalgo planning to create three public bathing areas in its waters next year. One of the features of the sewerage system - which dates back to the mid-19th century - is that it collects sewage, domestic waste and rainwater in the same underground tunnels before sending them to treatment plants.

During heavy rainfall, the system is overwhelmed and valves are opened, releasing excess water containing untreated sewage directly into the Seine. In the 1990s, this resulted in around 20 million cubic metres of dirty water containing sewage being discharged each year, according to figures from the mayor's office.

In recent years, following a multi-decade investment and modernisation programme, the figure has fallen to around two million cubic metres. Discharges currently average about 12 times a year at present, but city officials say the new facility should reduce that to two.

A major storm or series of heavy rains could still lead to the cancellation of the Olympic swimming events, but chief organiser Tony Estanguet stressed that there were contingency plans were in place, including the ability to delay the races by several days if necessary.

"With all the measures that have been put in place and the planning that has been done, we are very confident that the competitions will go ahead," he told reporters as he inspected the storm water facility. Three Olympic trials were cancelled last summer after heavy rain. Some swimmers, including Brazilian Olympic champion Ana Marcela Cunha, have called for a Plan B in case the Seine is too dirty.

Olympic open water swimming has often been plagued by pollution concerns. At the end of the Tokyo Olympic test event in 2019, swimmers protested about the quality of the water in Tokyo Bay, and at the 2016 Rio Olympics, the prospect of swimming in polluted Guanabara Bay also made headlines.

Hidalgo has promised to take a dip in the Seine with President Macron before the Games to demonstrate its safety, just over a century after public swimming was banned there in 1923. "We'll give you the date. We'll set a time frame to do it, because in June you can have good weather but you can also have storms," she said.