An oasis is a fertile place in the desert caused by the presence of water from underground springs or rivers, or even by manmade wells. Date palms, fig trees, rice, wheat, cotton, vegetables, alfalfa, barley and wheat are all examples of commonly grown plants in oases. To desert cultures of old, oases were of vital importance as sites for agriculture, trading and manufacturing, and the cultures that frequented oases often established rigid codes of conduct regarding them. No one would be allowed to despoil an oasis, and if the unthinkable should happen, the penalty was often death. Most oases in the Arabian Peninsula became sites for shrines, and this added sanctity also brought about a further code of conduct, namely no one could shed blood at the oasis where the shrine was located. This allowed the often-warring tribes of Arabia neutral territory in which to trade or conduct peace negotiations. The Silk Road was mostly a series of oases across the forbidding territory of central Asia between China and the Middle East. Mecca, Cairo and Baghdad are examples of cities that originated because of an oasis, although all of these have grown much larger due to modern influences. In modern times, irrigation has also created oases, the most notable (and large) being the Imperial Valley in California.