Cyclocross is a bike-racing discipline that combines road racing with off-road surfaces. Competitors pedal, run, and carry their bikes around a closed course, usually for 30-60 minutes. Oh, and there's mud - lots of mud.
Riders dismount to jump the barriers, then remount their bikes in one fluid motion. Done right, there's no loss of momentum, and it's a thing of grace and beauty.
Mud saps energy, gums up the brakes and gears, and makes the bike feel like a hundred pounds.