During the first century, CE Greek sailors pioneered the Indian Ocean trade routes, using the Monsoon winds to take their sailing ships to and from the subcontinent. Abundant archaeological evidence in India points to the extent of this trade. Unfortunately for the towns and kingdoms of southern Arabia, the Greek merchant ships could by-pass the southern Arabian kingdoms in a way that camel caravans could not, and as this maritime trade gathers pace, the slower and more expensive overland routes have declined. With the decline of the overland trade has come to a decline in the fortunes of the southern Arabian kingdoms as well as the desert nomads. These latter are now occupying large areas of previously fertile farmland as the centuries-old irrigation systems become degraded. In the north, the Nabatean kingdom has been annexed to the Roman empire.