A note came in recently from a homeowner who had a leaking pressure release valve on her home's hot water heater. She was collecting about 2 cups of leaking water a day from the faulty valve. She asked if she should wait until the problem worsened or if she should be calling a plumber. This Mythbusters video was sent as a warning. It may seem a bit alarmist at first, but the pressure release valve (also called a pressure relief valve) on a hot water heater is there to ensure that pressure within the tank doesn't build up too high. Apparently there are folks out there who don't believe that a hot water tank can "explode" if pressure is allowed to build.
Thank you, Mythbusters, for setting those non-believers straight.
Needless to say, the woman with the leaking valve was encouraged to call a plumber straightaway.
Note: Changing a faulty pressure relief valve is a perfectly suitable DIY job for those comfortable with such handy projects. There are a couple important safety steps to follow, like shutting off the electricity, gas and water supply to the water heater.
If you still aren't convinced of the dangers of a faulty pressure release valve, check out these images of the aftermath of such an explosion.