Above the South China Sea (CNN) Take-off is swift on the U.S. Navy's P8-A Poseidon -- there's no slow roll down the runway on military flights -- and within minutes, the turquoise blue of the South China Sea comes into view from the window of America's most advanced surveillance and sub-hunting aircraft.

The P8 crew, composed of more than a dozen naval aviators, invited me into the cockpit to watch and to listen. CNN last week first reported on this secret surveillance mission, which eight times received warnings from the Chinese navy to leave the contested area. This is a behind-the-scenes look at the journey.

Approaching the P8-A Poseidon on the tarmac, it's easy to mistake the sophisticated aircraft for one of the many other Boeing 737 planes around it. The P8 is based on a 737 frame, but the similarities end there.

Peeking out of the fuselage are numerous antennas, domes and camera wells, plus a bomb bay for sub-hunting torpedoes and spaces under the wings for Harpoon air-to-surface missiles. Inside, the jet is home to an array of advanced intelligence-gathering equipment. I have the feeling of entering a CIA listening station in the sky.

It's no accident that the first P8s -- only 18 months old -- were deployed to Asia. The P8 is one expression of the Obama administration's pivot to Asia, and its principle mission here is keeping a watchful eye on China.

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Once we're aboard, we sit in on the flight crew's "delta briefing," the final briefing before takeoff. The aircraft's commander, Lt. Cmdr. Matt Simpson, lays out the mission plan. The P8 will leave from Clark Air Base in the Philippines and fly some 460 miles west to three reefs -- Fiery Cross, Subi and Mischief -- which Beijing has rapidly been transforming into expansive islands and, the U.S. fears, soon-to-be permanent military stations 600 miles from China's coastline.

From the air, the waters appear quiet and peaceful, but 60% of world trade traffics through here. And underneath the sea floor are believed to be enormous deposits of oil and gas. That helps explain the international free-for-all underway to claim an array of barely visible reefs and shoals.

It's not only China but also the Philippines, Vietnam, Malaysia, Indonesia and Brunei that are involved. Though the coastlines of the other countries are all closer than China's, they are still more than 200 miles away, far from their territorial waters, which by international law extend only 12 miles out from shore.

America's own coastline is, of course, thousands of miles away, but the U.S. is still very much in the mix. It's the defender, in effect, of free passage through international waters for all international cargo ships and, crucially, the U.S. Navy.

Forty-five minutes into the flight, the first mission target comes into view: Subi Reef. More than two dozen Chinese dredgers fill the lagoon, pumping sand from the ocean floor in huge plumes onto the surface, slowly and gradually building up an island from scratch. The scale and pace of their work is mesmerizing. In two years, China has expanded the surface area of reefs it claims in the South China Sea from just five acres to more than 2,000.

The P8 crew expect the Chinese navy to warn them away as we approach. Usually these "challenges," as they refer to them, come once the jet is within a few miles, and indeed, the radio soon relays a voice speaking in Chinese-accented English: "This is the Chinese navy. This is the Chinese navy ... Please leave immediately to avoid misunderstanding."

From both sides, the communication is calm and, mostly, professional. The American pilot quickly responds from a well-rehearsed script explaining that this is a U.S. aircraft operating in international airspace over international waters.

Still, as the Chinese navy radio operators come back on the line to repeat their warning, I can hear frustration building in their voices. In one case, the operator's voice grows louder and louder, ending in a high-pitched "You go!" The Chinese give warnings like this eight times during the flight.

It turns out there are also civilian aircraft in the vicinity. A Delta Air Lines flight on that same frequency hears the Chinese challenge, and then pipes into the frequency to ask what's going on.

The Chinese navy radio operator identifies himself, possibly to reassure them. The flight crew notes that this can be a very nerve-racking experience for civilian aircraft in the area.

A depiction of where exactly China is building islands out of nothing, 600 miles off the coast of its mainland.

For now, these Chinese challenges are surface-to-air, coming from new early-warning radar stations they have positioned on the islands or from Chinese navy ships patrolling them. These reefs are too far from air bases in China for Chinese aircraft to intercept the P8 and start a potential confrontation in the air, but that may change if and when these islands have working airstrips. Fact is, China is already on its way.

As we leave Subi Reef, we approach Fiery Cross Reef a few minutes travel away. Here China has made the most extensive progress. What used to be a rocky reef barely visible above the waves is now equipped with a nearly completed runway, a tower, an early warning radar station and barracks for Chinese soldiers and sailors.

Meanwhile, Chinese dredgers continue to dig away at a deep-water harbor for Chinese ships. This is what some call China's "unsinkable aircraft carrier." If China continues to militarize the islands, as the U.S. fears, the three reefs will form strategic Chinese military bases right in the middle of some of the most contested waters in the world.

On board the P8, the mood is calm and confident. These crews have been flying these flights for months now. But the more China builds, the U.S. sailors tell me, the more aggressively the Chinese navy is challenging them.

CNN has learned that the U.S. is considering flying its aircraft -- and sailing its Navy warships -- even closer to the islands to further demonstrate that the U.S. does not recognize China's claims. The lowest we fly on this mission is 15,000 feet. But that could soon change.

And what happens if China starts deploying its aircraft to these waters? The airstrip on Fiery Cross Reef is already long enough to accommodate every military aircraft in the Chinese arsenal. P8 flights hundreds of miles closer to the mainland are already intercepted by Chinese fighters. Last fall, in one such encounter, the Chinese pilot came dangerously close.

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The prospect of U.S.-Chinese collisions in the air are truly alarming, as past events have demonstrated. In 2001, a Chinese jet collided with a U.S. EP-3, the P8's predecessor, over Hainan Island, right off the Chinese mainland, leading to a major crisis as the damaged U.S. plane barely managed a safe landing on Chinese territory.

The P8 flight crew speaks confidently and calmly about their mission. But they know they are in the midst of a major and contentious potential conflict. Chinese protests are becoming more stern. U.S. surveillance flights and naval patrols are becoming more aggressive. And the two countries' positions are, for now, irreconcilable.

China views these islands as its sovereign territory and describes its commitment to them as "unshakable." The U.S. views the area as international waters and airspace. Aboard the P8, it is difficult to see how those differences are resolved.