Sludge was a comic book series from Malibu Comics, set in the Ultraverse. It was created by Steve Gerber, Gary Martin and Aaron Lopresti. Sludge ran for only twelve issues, with one special: Sludge: Red X-Mas. A second special, Sludge: Swamp of Souls, was planned but never completed. There exists a connection between the chemicals that transformed Frank Hoag into Sludge and Dr. Gross' research. Dr. Gross conducted the experiments that allowed Kevin Green to transform into Prime.