According to this article on Core77, wheelchair users are no longer limited on riding just on perfectly flat, smooth surfaces. Going off-road and even underwater is now perfectly possible.

Your average car is designed to drive on a paved road, and your average wheelchair is designed to roll across smooth surfaces. But just as 4x4 manufacturers design vehicles for alternate terrains, a host of inventors have begun wondering why wheelchairs oughtn't do the same. Handicapped folks, after all, do not live only in cities; and those in rural areas may not even have paved driveways.

A company called TC Mobility produces the Tank Chair, an off-road, all-weather wheelchair that "conquers streams, mud, snow, sand, and gravel, allowing you to get back to nature. Using rubber tracks and high-torque electric motors, TankChair will take you anywhere and back."