Have you ever hear that rumbling in the far distance or sudden crash of sound during a thunderstorm? That is called thunder! It is caused by extreme heat fromt he lightning going through the air from the cloud to the ground. The air is heated to 15,000 to 60,000 F in less then a second. From this rapid heat the air rapidly expand. This rapid expansion of the air creates a shock wave which is heard and identified as thunder.
Close lightning strikes will sound like a loud bang, crack or snap. However, if it is far away it will make a long rumbling noise for a long period of time as the sound arrives at different times due to the length of the lightning flash. On a quiet still night the thunder can be heard from as far as 10 miles away.
In the above picture is a shot taken during a storm in the far off distance that had rolling thunder.....because thunder is a sound, we can not see thunder.