How to cracks and craters get into the street pavement?

I've always wondered how streets get so bad to the point of huge craters in every section of the street. The tires on cars are made out of rubber, so what is happening to the street that makes it so bad at times? Around my house I can't even drive faster than 25mph without the fear of popping one of my tires!How to cracks and craters get into the street pavement?
Its the stuff thats underneath the pavement that causes it.





Normally they put stone, gravel, clay, and concrete under streets. If its not built right, then cracks develop.