Aboard a US Navy P-8A Poseidon reconnaissance plane, CNN got a view from 16,500 feet of low-lying coral reefs turned into garrisons with five-story buildings, large radar installations, power plants and runways sturdy enough to carry large military aircraft.
During the flight the crew received six separate warnings from the Chinese military, telling them they were inside Chinese territory and urging them to leave.
“Leave immediately and keep out to avoid any misunderstanding,” a voice said.
The US Navy jet flew past four key artificial islands in the Spratly chain where China has built up fortifications: Subi Reef, Fiery Cross Reef, Johnson Reef and Mischief Reef.
On Subi Reef, the Poseidon’s sensors picked up 86 vessels, including Chinese coast guard ships, moored in a giant lagoon, while on Fiery Cross Reef rows of hangers stood alongside a lengthy runway.
“It was surprising to see airports in the middle of the ocean,” said Lt. Lauren Callen, who was leading the air combat crew aboard the Navy flight.
Each time the aircraft was challenged by Chinese military, the US Navy crew’s response was the same.
“I am a sovereign immune United States naval aircraft conducting lawful military activities beyond the national airspace of any coastal state,” the response said.
“In exercising these rights guaranteed by international law, I am operating with due regard for the rights and duties of all states.”
CNN has reached out to the Chinese Ministry of Foreign Affairs for comment.
Vietnam, the Philippines, Taiwan, Indonesia, Malaysia and Brunei all claim overlapping portions of the sea which spans 3.6 million square kilometer (1.4 million square miles), but the most far-reaching claims have been made by China.
Despite this however, little has changed in China’s approach to the region in recent years.
To reinforce its claims of sovereignty, Beijing has been reclaiming land on and around reefs and shoals to construct artificial islands which are then militarized with airfields and radar equipment.
Since then, Beijing’s island building in the contested waters has moved forward at a rapid pace.
Flying over Fiery Cross Reef on Friday, a five-story building was visible, as well as a large radar installation, which looked like neatly arranged golf balls on the Navy plane’s infrared camera.
Though no Chinese missiles were seen on Friday’s flight over the South China Sea, Navy officers said some of the structures seen could potentially be used to house them.
Cmdr. Chris Purcell, who leads Patrol Squadron Four which undertook Friday’s mission, said the US has been doing these flights for five decades and they show US commitment to maintaining free passage in international waters.
“The reason we’re here hasn’t changed,” Purcell said. “The reason (the Chinese) are here has changed.”
China claims US sparking militarization
Beijing says its growing military presence in the South China Sea is necessary to protect its sovereignty, blaming Washington and its allies for tensions in the region.
Beijing points to the regular US Navy patrols and flyovers of the South China Sea as an example of US militarization and provocations, and a justification for the increased Chinese military presence.
In the past year, the US has stepped up freedom of navigation operations in the region, sailing US naval vessels within miles of China’s artificial islands across the South China Sea.
Time may be running out to effectively challenge China’s claims in the South China Sea however.
“China is now capable of controlling the South China Sea in all scenarios short of war with the United States,” Davidson said.
CNN’s Steven Jiang contributed to this article.