Land Titanic. Leela's grandmother working as a drain angel in the

"Drain angel" was the designation of some child mutants, such as Leela's grandmother, cleaning the Land Titanic when it sunk in 2912 (6ACV12).

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Quotes

Leela's grandmother: I was just a young girl. [A flashback starts. She is cleaning a pipe.] Drain angels, they called us.