Answer: The mid-ocean ridge system is the most extensive chain of mountains on earth, but more than 90% of this mountain range lies in the deep ocean. The mid-ocean ridge wraps around the globe for more than 65,000 km like the seam of a baseball. The average depth to the crest (top) of the ridge is 2500 m, but it rises above sea-level in Iceland and is more than 4000 m deep in the Cayman Trough.