The severity of a natural gas explosion depends on the volume of accumulated gas and the amount of pressure it is under. The higher the pressure and volume, the greater the hazard.
If the gas is trapped underground or in an enclosed space, the concentration could be very high. In fact, natural gas explosions have been known to generate fireballs as high as 1,000 feet and to reduce multiple buildings to rubble.
If a leak is outdoors, the gas that is released will often disperse on its own. However, even outdoor gas leaks can enter enclosed spaces and produce an explosion if the right concentration of gas finds an ignition source.