Development of highly sophisticated seismic recorders and precision depth recorders in the 1950s led to the discovery in the early 1960s that the Mid-Atlantic Ridge, a vast, sinuous undersea mountain chain bisecting the Atlantic Ocean, was in fact only a small segment of a globe-girdling undersea mountain system some 40,000 mi (64,000 km) in length. In many locations, this mid-ocean ridge was found to contain a gigantic cleft, or rift, 20 to 30 mi (32–48 km) wide and c.1 mi (1.6 km) deep, extending along the crest of the ridge. The ridge itself does not form a smooth path, but is instead offset in many places. The offsets are called fracture zones, or transform faults. The ridge crest and its associated transform faults are the locus of nearly all shallow earthquakes occurring in mid-ocean areas. Continued study of the mid-ocean ridges is a major component of U.S. research in the global oceans.