It seems probable that before around 4000 BC the Arabian climate was somewhat wetter that today, benefitting from a monsoon system that has since moved south. During the late fourth millennium BC permanent settlements began to appear, and inhabitants adjusted to the emerging dryer conditions. In south-west Arabia (modern Yemen) a moister climate supported several kingdoms during the second and first millennia BC. The most famos of these is Sheba, the kingdom of the biblical Queen of Sheba. These societies used a combination of trade in spices and the natural resources of the region, including aromatics such as frankincense and myrrh, to build wealthy kingdoms. Mārib, the Sabaean capital, was well positioned to tap into Mediterranean as well as Near Eastern trade, and in kingdoms to the east, in what is today Oman, trading links with Mesopotamia, Persia and even India were possible. The area was never a part of the Assyrian or Persian empires, and even Babylonian control of north-west Arabia seems to have been relatively short-lived. Later Roman attempts to control the region's lucrative trade foundered. This impenetrability to foreign armies doubtless augmented ancient rulers' bargaining power in the spice and incense trade.