The Maintenance Tunnels are a mythical location located in downtown Los Santos, with two entrances. One at the far end of the Los Santos water reservoir and one located beneath an underpass. The tunnels are used during the first heist as a escape route.
The Maintenance Tunnel myth involves a ghost, who is thought to be haunting the tunnels. Players have reported hearing voices and strange noises from an unknown source while traveling within the area.
In one part of the tunnels, near the unfinished station, a demonic, shrill-sounding laughter can be heard lightly echoing. If the player sticks around in the area for a while longer, it can be heard once again. Multiple players and myth hunters have confirmed this, and as of yet, no explanation has been given. This has rumored to be related to Ratman due to the high pitch of the laughter. This could be an Easter egg by Rockstar, poking fun at the endless stream of Ratman reports in GTA IV.
A strange breathing noise occurs whenever the player enters the water reservoir entrance to the tunnels and starts down inside. It always happens, but is hard to hear the noise, so the volume must be turned up. This occurs in both GTA V and GTA Online. Some believe it may be Ratman or a ghost's breathing as they are watching the player(s.)
Ratman is also rumored to inhabit the tunnels and sewers due to the environment in the tunnels, and the fact it is a major location for normal rats. There has been no 100% proof of this though, and it is most likely a continuation of the Grand Theft Auto IV myth.
Some players also claim of other odd occurrences, such as a strange beating noise, most likely a metro passing, and even reports of health slowly decreasing, similarly to Panic in the Woods.