The seabed is that area of the Earth which has been researched the least. However, we know, through sound wave technology, that there are deep valleys, high mountain ranges, steep slopes, and flat regions on the seabed. We also know that the seabed undergoes continuous changes. Underwater volcanic mountain ridges form at place where two tectonic plates move away from each other, and these ridges grow every year by a few centimetres. Generally, the bed of the flat seas, such as the coral reefs (page 78), is densely populated with plants and animals. In contrast, the deeper regions in the seabed are often covered by a layer of sand, dust, and dead organisms from the rivers, which is several hundred metres thick.