The Sewers are a minor location in The Amazing World of Gumball. They are quite obviously the waste disposal system that runs underneath Elmore. Like most fictional sewer systems, the Elmore one is very spacious and complex.
As shown in "The Secret," water that is sold to consumers in Elmore originates in the sewers.
In reality, many of the sewer tunnels seen in the series appear to be the Greenwich Foot Tunnel in London.