How do Bowman, Kestrel, and Mumpo escape the city in the novel, The Wind Singer?

Bowman, Kestrel, and Mumpo escape the city through the The Underlake, which is populated by people who cover themselves with mud, and live off and in the trash from the world above. The Underlake is a subterranean region beneath Aramanth, into which the citizens deposit their filth and waste.