Cuarteron Reef (Huayang Jiao 华阳礁 in Chinese; Calderon Reef in Filipino; đá Châu Viên in Vietnamese) is a reef at the east end of the London Reefs in the Spratly Islands of the South China Sea. It is controlled by China as part of Sansha, and claimed by the Philippines as part of Kalayaan, Palawan. The reef is 3 nautical miles long and has an area of 8 square kilometres.
On 12 July 2016, the tribunal of the Permanent Court of Arbitration concluded that Cuarteron Reef contains, within the meaning of Article 121(1) of the Convention, naturally formed areas of land, surrounded by water, which are above water at high tide. However, for purposes of Article 121(3) of the Convention, the high-tide features at Cuarteron Reef is rocks that cannot sustain human habitation or economic life of their own and accordingly shall be entitled to 12nm of territorial sea measured from its baseline but have no exclusive economic zone or continental shelf. 
Carteron reef has a supply platform and a reef fortress. In 2015 the Philippine government release what it claimed were photos of a six-story facility being built by the Chinese government on the reef. Also in 2015, China completed the construction of a 50 metre high lighthouse on the reef. By late 2015, China had expanded the reef's usable area to 0.23 square kilometres (0.089 sq mi), reportedly including in the process two helicopter pads, and possibly gun, radar and missile emplacements. As of 2016, China was reportedly building a high-power radar installation on the reef.
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