“The massive mid-ocean ridge system is a continuous range of underwater volcanoes that wraps around the globe like seams on a baseball, stretching nearly 65,000 kilometers (40,390 miles),” according to the National Oceanic and Atmospheric Administration (NOAA).
“The ridge system forms the longest and largest mountain range on Earth, winding its way between the continents,” NOAA continues.
The majority of the system is underwater, with an average water depth to the top of the ridge of 2,500 meters (8,200 feet).”
“The mid-ocean ridge is the most extensive chain of mountains on Earth, with more than 90 percent of the mountain range lying in the deep ocean.
The nearly continuous, global mid-ocean ridge system is clearly visible on this map of global topography above and below sea level. 
“Despite being such prominent feature on our planet, much of the mid-ocean ridge system remains a mystery.
While we have mapped about half of the global mid-ocean ridge in high resolution, less than one percent of the mid-ocean ridge has been explored in detail using submersibles or remotely operated vehicles.”
The Mid-Atlantic Ridge, which is only a part of the mid-ocean ridge, extends for about 10,000 miles (16,000 km) in a curving path from the Arctic Ocean to near the southern tip of Africa.
The ridge is equidistant between the continents on either side of it.
... Due to Iceland’s location on the Mid-Atlantic Ridge and its location over a hot spot, the country boasts a high concentration of active volcanoes ...
“Over the past 500 years ... Iceland’s volcanoes have produced ONE THIRD of the TOTAL GLOBAL LAVA OUTPUT.”
(The total global lava output that we are aware of).
... More than forty thousand miles of underwater volcanoes – who knows how many are active?
– spewing out red-hot magma (as much as 2000 degrees F), and we can’t figure out what is heating our seas? "