Sewer Boss Chests are items that can be obtained by defeating the Sludge boss that can appear in the Sewer. It, like other Main Hub boss chests, can drop gems, crate keys, hoverboards and a pet. Sewer Boss Chests cannot be burned at the Incinerator in the Junkyard.
As of v1.103.0, this item can be converted by the Main Hub Gumball Machine into Gum. The exchange rate is 1:1.