A unique trekking trail of Nepal is Upper Mustang Trek. Hidden behind the mountains of Annapurna (8,091 m) and Dhaulagiri (8,167 m), the ancient kingdom of Mustang lies on the border with Tibet. It is very dry because of the rain shadow of the surrounding high mountains. Kali Gandaki rises in the mountains of Mustang. Going south through Mustang, it forms the deepest trench on Earth.