Surrounded in mystery and tales of demons, the Well of Barhout in Yemen's east -- known as the "Well of Hell" -- is a little-understood natural wonder. Closer to the border with Oman than to the capital Sanaa 1,300 kilometres (800 miles) away, the giant hole in the desert of Al-Mahra province is 30 metres wide and thought to be anywhere between 100 and 250 metres deep.