Drones could take over the tricky inspection work within two years, if a scheme in Barcelona gets the go-ahead.
The plan is to launch a fleet of flying "sewer drones" which would inspect the state of the city's sewers, as well as measuring air and water quality and keeping track of the state of the walls and blockages.
The drones would, at least in part, replace sewer workers, who do an unpleasant and, often, dangerous job patrolling the city's dirty underbelly.
As well as limiting the risks to people working in the sewers, the drones would provide more precise inspections and even reduce the cost of cleaning, according to the project's founders.
"This is the first time we have researched the profitability of using drones in this kind of activity," Daniel Serrano, of Eurecat, one of the companies taking part in the project, told Spain's El País newspaper.
Other groups making up the team include German robotics company Ibak and drone operator Simtech Design, as well as the FCC, which currently looks after Barcelona's 1,500km long network of sewers.
Drones would be controlled from a van on the surface and people would only have to descend into the sewers to change the flying machines' batteries.
The first trials of the project took place on December 1st, which will be followed by more rigorous testing in the coming months.
But there could be a hitch. The drones that are currently available are too big to patrol the majority of the narrow tunnels of Barcelona's sewers: experts estimate that at the moment they could only fit through 39 percent of the network of tunnels.
The challenge now for the companies behind the project is to make smaller, lighter drones, that could fit through the narrow gaps in the sewer system and could work for longer periods without needing a change of batteries.
Sewer workers currently use cables and tiny robots to access the hard-to-reach spaces in the underground tunnels. Those robots can access around 55 percent of Barcelona's sewerage network.