Unidentified hexagonal structures at Fiery Cross, Subi, and Mischief reefs. CSIS/AMTI/Digital Globe/Amanda Macias/Business Insider In addition to China's lightning-fast construction of military aircraft hangars on reefs in the South China Sea, mysterious facilities have also emerged.

Satellite imagery published by the Asia Maritime Transparency Initiative (AMTI), a unit of the Center for Strategic and International Studies, shows several unidentified hexagonal structures on Fiery Cross, Subi, and Mischief reefs.

The formations are always oriented toward the sea and started to appear in May, according to experts at AMTI.

"I'm afraid I can't comment just yet," Gregory Poling, director of AMTI, told Business Insider about the hexagonal designs. "But the reason we put them out like this is to collect opinions from other experts in the field."

It has been nearly a month since the Hague-based Permanent Court of Arbitration invalidated Beijing's vast territorial claims in the South China Sea. All the while, Beijing has maintained the ruling has no bearing and continues to build in the region.

All photos republished with permission from the Center for Strategic and International Studies.