The lake of the Crescent Moon which was given to Biggles as a navigation landmark in Biggles in the Gobi is an actual location and refers to a spring 6 km south of the city of Tunhwang (now Dunhuang). The lake is more commonly called Crescent Lake (In Chinese, 月牙泉 , Yueya Quan in pinyin transliteration).
As described in the book, it is a lake of a crescent shape whose waters take on a brilliant blue colour standing out starkly in the midst of a desolate desert landscape, surrounded by huge sand dunes. Said to be almost 2000 years old, the lake is about 200 metres long by 20 metres wide. In Biggles' time, the lake had a depth of some 4-5 metres but it has been shrinking and today it has a depth of about 0.8 metres. In 2006, the authorities took action to refill the lake and the level has been said to be rising again. Next to the lake is a chinese style pagoda, and as described in the book, a line of poplars.
The lake is, today, a popular tourist attraction near the town of Dunhuang where tourists come for camel rides and to climb the surrounding sand dunes.
As described by the Air-Commodore, the Crescent Moon Lake, a patch of vivid blue and green in the midst of the desolate Gobi Desert landscape made a good navigation landmark.