Unidentified hexagonal structures at Fiery Cross, Subi, and Mischief reefs. CSIS/AMTI/Digital Globe/Amanda Macias/Business Insider WASHINGTON, DC — Experts still aren't quite sure what these mysterious hexagonal facilities in the South China Sea are.

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

"I'm afraid we still don't know for sure what the structures are, beyond saying that they appear defensive in nature," Gregory Poling, director of AMTI, told Business Insider about the hexagonal designs.

Bonnie Glaser, senior adviser for Asia and the director of the China Power Project at CSIS, echoed Poling, telling Business Insider, "there has been some speculation that they might be SAM sites."

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

It has been a little over five months 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.