Something astonishing is happening in the South China Sea, and there are satellite photos to prove it. The Chinese government has been sending construction equipment to a string of small islands in disputed waters and quietly making them bigger.

The effect is pretty dramatic — drag the slider below to see how things have changed at Fiery Cross Reef in the Spratly Islands, which are claimed by China and several other Asian countries. The first photo shows the reef last August. It’s totally underwater; the only habitable area is a small concrete platform built by the Chinese navy. Now look at the second photo, showing the same reef in January. It’s now an artificial island more than 3,000 yards long. Defense analysts from IHS Jane’s, which released the photos taken by a commercial satellite wing of Airbus, say it looks capable of housing an airstrip.

The South China Sea has tremendous strategic importance for its location between several countries and for its potential resources, which is why regional powers have competed for centuries over who controls it. At present, six nations claim sovereignty over overlapping parts of the sea and its many islands: China, Vietnam, the Philippines, Taiwan, Malaysia, and Brunei. That’s what makes China’s new "sandcastles," an effort to build up its military infrastructure in the Spratly Islands and exercise more control over the surrounding waters, so controversial.

US Assistant Secretary of State Daniel Russel urged China in February to stop the work, calling it "destabilizing"; China told America in no uncertain terms to back off.

What's the point of claiming these islands?

Whoever holds the South China Sea islands holds a highly strategic base between several Asian states. If you have a military presence here, you can use it to project power across this region and even implicitly threaten nearby countries that could potentially threaten you — and in this unpredictable part of the world, that can't be ruled out.

There’s also the possibility that these largely unexplored waters could harbor major oil and gas reserves. China announced a significant gas find nearby in 2014, for example.

The key thing here is that rights over waters — and the natural resources beneath them — are tied under international law to whether your coastline is nearby. So if you can claim ownership of islands in the South China Sea, you can potentially claim any lucrative resources in the waters around them.

Why China's neighbors are so worried about this

China's real estate grab at sea is adding to fears among its neighbors over how the country's rapid rise is altering the regional order.

Their fear is that China, as it grows in wealth and power, will define its role as that of a regional bully that pushes around weaker states. The construction work in the Spratly Islands seems to back that up fear. It sends the message, "We’re going to do what we want here. Who’s going to stop us?"

Over the weekend, Chinese Foreign Minister Wang Yi defended the project as "work on our own home" and indicated that the island-building isn’t part of some grand plan to build a new world order. "We don't want to upset the boat," Wang told reporters.

But this episode is just the latest in a series showing China’s growing aggressiveness in the South China Sea. In 2012, China seized the Scarborough Shoal area of the sea after a three-month standoff with Filipino coastguards. And last year Beijing placed an oil rig near the Paracels, another disputed island chain, sparking deadly riots in Vietnam.

This construction is bringing a Chinese military buildup

The Chinese government has openly admitted to carrying out land reclamation at six reefs in the Spratlys. It can’t really deny it — the satellite photos show the startling scale of the changes.

But there's more going on here than just land reclamation. The photos suggest China is using these islands to build up its military presence in the South China Sea. It's not just the airstrip that seems to be under construction at Fiery Cross. The expanded Gaven Reef has a helipad and a new anti-aircraft tower. Hughes Reef, another tiny land formation that has grown into a full-blown artificial island, now has two piers and a cement plant.

China’s not the only country to have placed military facilities on South China Sea islands — four other countries have done so, with the exception of Brunei. But the scale of China's efforts is much greater, and is worrying to its neighbors who already fear China is becoming a bully in the region.

There's an international legal fight over what counts as rocks versus islands

Land reclamation is not in itself illegal, and Beijing points out that it’s not alone in using this tactic — Malaysia and Vietnam have both reclaimed land in the Spratlys.

But the issue is likely to come up at a UN tribunal where the Philippines is pursuing China over the wider territorial dispute. That’s because of a distinction that seems a little ridiculous but actually turns out to matter: the difference between rocks and islands.

Under the United Nations Convention on the Law of the Sea, the international agreement the Philippines is citing, "islands" have much more significant territorial rights than do "rocks." Holders of islands gain what’s called an Exclusive Economic Zone, which infers key rights within 200 nautical miles of the coastline, including exclusive access to energy exploration.

China says the reefs it’s expanding are islands; the Philippines insists they’re a combination of rocks and "low-tide elevations," which carry fewer privileges.

As Gregory Poling, a fellow at the Center for Strategic and International Studies, explains, the reason this matters is that geographical surveys in the South China Sea have traditionally been really poor. And the more China builds, the less obvious it is whether these snippets of land started off as rocks or as islands. Ingenious!

In any case, whatever the verdict at the UN, China is widely expected to ignore the results and merrily carry on with its building work.

Why China's claim extends so far away from its coast

China claims about 90 percent of the South China Sea — everything within a massive loop called nine-dash line . Taiwan, despite being tiny, claims the same area. That’s because it was the Nationalist government of China (which later was expelled from mainland China and became Taiwan’s government) that announced this claim in 1947.

It may seem crazy for China to insist it’s the rightful owner of waters so close to the coastlines of other countries like Vietnam and the Philippines, hundreds of miles south of its own mainland. Beijing uses history to back up its claim — saying early Chinese fishermen held the Spratlys and Paracels more than 2,000 years ago and the islands are still rightfully China’s — but it's a bit of a stretch.

Vietnam has also appealed to history, pointing to its rule of the islands in the 17th century. The Philippines argues geographical proximity; Malaysia and Brunei also claim parts of the sea that they say fall within their Economic Exclusion Zones.

The big concern: the risk of violence, intended or accidental

These competing claims have led to violence before, and could again, whether through an intentional attack or by an unwanted escalation.

China seized the Paracels from Vietnam in 1974, killing more than 70 Vietnamese troops. Beijing’s increasingly bullish tactics have raised fears of another outright confrontation in these waters, which is home to some of the busiest international shipping routes in the world.

Making things all the more volatile, the US has repeatedly reiterated its intention to adhere to treaty obligations to come to the defense of the Philippines, an ally, in the case of a conflict. China sees this as American meddling in its backyard.

China's island exploits in the South China Sea, while significant in their own right, also speak to the country's growing regional ambitions, and to the future of power politics in this increasingly important — and tense — part of the world.

Correction: this post originally said Greg Poling worked for the Asia Maritime Transparency Initiative, which was founded by the Center for Strategic and International Studies (CSIS). He is a fellow with the Sumitro Chair for Southeast Asia Studies and the Pacific Partners Initiative at CSIS.