Hydroplaning is caused by driving too fast on wet road surfaces. When driving at speeds of less than about 35 miles per hour (56 kilometers per hour), your tires will brush off the water on the road's surface in much the same way window wipers move the water on your windshield.
The faster you drive, however, the harder it is for your tires to dissipate the volume of water. A film of water can form between your tires and the roadway, and your car can actually ride on top of this film, losing all traction. If this happens, you may be unable to brake, accelerate, or change direction.
Take your foot off the accelerator and let the car slow down. As your speed decreases, the tire's footprint (the amount of the tire's tread touching the road) increases, and you'll slowly get better traction.
Note: Hydroplaning is less likely to occur if your tires have deep treads. These treads will let the water escape from under the tires. If the depth of the water on the road is deeper than the depth of the treads, hydroplaning can occur.