A mountain range with a total length of 65,000 kilometers runs through all the oceans. It marks the boundaries of tectonic plates. Through the gap between the plates, material from the Earth’s interior emerges, forming new seafloor, building up submarine mountains and spreading the plates apart. Very often, these mid-ocean ridges are described as a huge, elongated volcano. But this comparison is only partly correct, because the material forming the new seafloor is not always magmatic. At some spreading centres, material from the Earth’s mantle reaches the surface without being melted. The percentage of seabed formed of this material has been previously unknown.