Hydra the Revenge, or simply Hydra, is a steel Floorless Coaster at Dorney Park & Wildwater Kingdom in Allentown, Pennsylvania. Manufactured by Bolliger & Mabillard, the coaster opened to the public on May 7, 2005. Hydra was built on the site of the former wooden coaster Hercules, which was closed and demolished at the end of the park's 2003 season. The ride's name comes from the Greek story where Hercules battled the Lernaean Hydra.

A Floorless Coaster, commonly known as a Floorless Roller Coaster, is a type of steel roller coaster manufactured by Bolliger & Mabillard where riders sit with no floor underneath them, allowing their feet to swing freely just above the track. Development of the Floorless Coaster model began between 1995 and 1996 with Medusa at Six Flags Great Adventure opening on April 2, 1999, making it the world's first Floorless Roller Coaster. Floorless Roller Coasters also tend to have 3 to 7 inversions incorporated in the layout of the coaster.