I assume you are talking about a fuel (gasoline) leak. A leak can happen when a fuel line or gas tank develops a hole, perhaps from rusting. It also can happen if a rubber seal deteriorates (something that is more common with alcohol added to the gasoline, especially on older vehicles. Other causes are failures of mechanical fuel pumps (older cars) damaged fuel filters and damaged fuel filler tubes (or gaskets, depending on the vehicle

Usually a rusted fuel line connecting to the tank

Trough holes on the tubes

a hole in the device