According to Science daily a mid-ocean ridge or mid-oceanic ridge is an "underwater mountain range, formed by plate tectonics." There are mid-ocean ridges in every ocean in the world although it was originally thought they were specific to the Atlantic Ocean. There is still much to be studied about the mid-oceanic ridge system as they were only discovered in the 1950's.
Researchers from Columbia University initially recorded data from the surface of the ocean and that data was later translated which led to the discovery of this new phenomena. Prior to this discovery the theory of continental drift was disregarded due to lack of evidence.
According to the New World Encyclopedia the mid-ocean ridge system is the largest mountain range in the world. In total this system is approximately 49,700 miles long and 40,400 of those miles are continuous and connected.