Curious crowds had gathered on the arched footbridges that criss-cross Paris’s favourite hipster hang-out, the Canal Saint-Martin, craning to get a look at the oddities poking out of the mysterious brown sludge below.

“Bloody hell, it’s a giant dustbin,” said Marie, a local office-worker, taking photos of the mountains of old wine-bottles, scores of office chairs, bikes, rolled-up carpets, wheelie suitcases and street signs stranded in the muddy pit.

Scores of the city’s Vélib hire bikes are among the items littering the bottom of the canal. Photograph: Yoan Valat/EPA

Paris’s picturesque Canal Saint-Martin, completed in 1825 but commenced on the orders of Napoleon, who wanted to reinforce the city’s drinking water supply, has in recent years become known as the favoured spot of Paris’s “Bobos” – bourgeois bohemians – wealthy, urban, young leftwingers who love to gather for summer evening aperitifs on the pavements along its banks. The canal is drained and cleaned once every 10–15 years, but when it happens, the bizarre items found at the bottom are one of the most curious and intriguing sights in the city, throwing up the mud-caked mysteries of Paris life.

An old ghetto blaster awaits its rescue. Photograph: Yoan Valat/EPA

On Monday, when workers began to slowly empty 90,000 cubic metres of water into the river Seine in a multimillion-euro cleaning operation that will last three months, they found one gun and police are poised for more. The last time the canal was drained, in 2001, the haul included two 75mm shells from the first world war, safes, gold coins, washing machines, at least one car and 40 tonnes of rubbish. In one earlier cleaning, 56 cars were fished out.

The full inventory is yet to be done – the fish have been evacuated and tractors will be brought in to drag away the vast number of items at the bottom. But the view from the bridges already reveals an extraordinary amount of debris that has been tipped into the canal in the past decade. There are scores of Paris’s Vélib hire bikes, lots of other bikes, myriad chairs of all descriptions, at least one motorbike, supermarket trolleys, shopping caddies, public dustbins, a fire-extinguisher, a children’s doll’s pushchair marooned in the middle of the canal, street signs, umbrellas and wheelie suitcases.

One of the many scooters found in the canal. Photograph: Charles Platiau/Reuters

“It’s like some kind of weird submarine treasure,” said Marc, 45, a self-employed local resident. “I just can’t believe the quantity of Vélibs in there. I guess they were stolen and thrown in afterwards. It’s bizarre,” he shrugged.

A vintage camera sits on the bottom, amid hundreds of discarded mussel shells. Photograph: SIPA/Rex/Shutterstock

Paris city hall, apart from warning people to resist trying to climb in to look for lost possessions that might have dropped in in recent years, are using the exercise to warn against littering the canal.

Bernard, 54, a public-sector worker, who had also been on the bridge to witness the last dredging in 2001, was aghast at the mountains of mud-caked wine-bottles and cans. “That’s Paris for you, it’s filthy,” he sighed. “The last time, I don’t remember seeing so much rubbish in it. I despair. The Bobos are using it as a dustbin.”