The Falador Sewers are a location in the first RuneScape novel, Betrayal at Falador. They cannot be accessed in-game, their location apparently hidden. The sewer entrance is described as being located near Falador Park. The sewers were abandoned since they were used as a temporary storage and meeting area for the White Knights some years before.
It is possible that a giant mole took up residence in the unpopulated sewers and further excavated them. This would explain why the only way to exit the Falador Mole Lair is to climb up out of a well which in medieval times was usually built near or accidentally built into a sewer of the city. The well is usually abandoned after the city inhabitants realise that the water they were drawing up out of the well was tainted.