South and west of Abu Dhabi, vast, rolling sand dunes merge into the Rub al-Khali (Empty Quarter) of Saudi Arabia. The desert area of Abu Dhabi includes two important oases with adequate underground water for permanent settlements and cultivation. The extensive Liwa Oasis is in the south near the undefined border with Saudi Arabia. About to the northeast of Liwa is the Al-Buraimi oasis, which extends on both sides of the Abu Dhabi-Oman border. Lake Zakher in Al Ain is a human-made lake near the border with Oman that was created from treated waste water.