For a short time, the water at Raystown Lake has been lowered for shoreline rehabilitation. This has exposed the lake bottom on the edges making for some prime fatbiking terrain, with the added bonus of riding the Allegrippis Trail System to connect shoreline adventures. If you are in the PA area it's worth going out to ride it before the water level rises and the lake bottom is covered again.

Here's my ride report with lots of pictures:
Suffering, Sweating, and Smiling: Confessions of an Ultra-Endurance Cyclist: Fat Tires at Raystown Lake

Rothrock Outfitters nearby rents Salsa Mukluks by the day