 While the land surface is continuously reused by societies, the sea bed was largely inaccessible until the advent of submarines, scuba equipment and remotely operated underwater vehicles (ROVs) in the twentieth century.Salvagers have operated in much earlier times, but much of the material was beyond the reach of anyone.Thus "Mary Rose" was subject to salvage from the sixteenth century and later, but a very large amount of material, buried in the sediments, remained to be found by maritime archaeologists of the twentieth century.