Terror From the Deep is the fifth and final Challenge Act of Seaside Hill Act 2 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the Dreamcast era and is playable only as Modern Sonic.
This Act take place within a highly modified version of Seaside Hill Act 2, where the pathway is frequently restricted to running across narrow sections of water.
In this Challenge Act, the player must reach the Goal within the given time limit. However, the route the character takes is altered so that they travel on water more often, leaving them vulnerable to getting swallowed by the Giant Chopper if they overstep the boundaries set by the Buoy. They must remain within the boundaries while still running quickly enough to stay above the water.
The player can find the Goal Ring in front of the temple section of the normal Act.