In certain areas of the world, some of the oldest sedimentary rocks on the ocean floor record an uninterrupted rain of sediment for over 180 million years. These rocks are mostly the skeletons of countless microscopic plankton and claysized particles. If the history of the Earth is to be better known, it is essential to look to the sediments and sedimentary rocks in the oceans where the record is more complete. Information about the ancient climates of the Earth, oceanic current patterns, and variations in volcanic activity in the past can be found within these rocks. Long coring pipes of the Deep Sea Drilling Project can recover a complete column of sediment and rock over a mile long.
Geophysical surveys provide a more sophisticated means of gathering data about the ocean floor.
Information about the nature of sediment-covered bedrock can be obtained by shipboard gravimeters, which measure the rocks' density, and by magnetometers, which measure their magnetic properties. Seismic surveys, using reflected sound waves, give valuable information about submarine topography and the thickness and folding and faulting of rocks that are covered with sediment. Seismic surveys are particularly useful for locating oil and gas deposits commonly found trapped in deep accumulations of sedimentary rocks. Seismic sound waves can be made by releasing compressed air, high voltage spark, mechanical clappers, or electronic pulse to create a spectrum of sonar frequencies. The profiles are recorded with great clarity and in many cases show structures as deep as 6 miles beneath the seafloor.