The law of the sea is clear and simple enough: a country's territorial sea extends to 12 nautical miles from shore and a country's exclusive economic zone - where the state has exclusive rights over natural resources - extends 200 miles.

The problem is that there are a lot of countries bordering the South China Sea - and their claims conflict. That's before you add in China's own extremely expansive claim - the nine-dash line which claims nearly the whole sea as theirs.

That was rejected by an international tribunal, a decision which China rejected. That's why features in the sea become so important - and so contentious.

Scarborough Shoal, referred to as Huangyan Dao by the Chinese, is in the Philippines' exclusive economic zone (EEZ). But as the reporting from our Asia team vividly shows, it is controlled by China.

Read more:How a Philippine ship ended up surrounded by 12 Chinese vessels The South China Sea is littered with similar small shoals, reefs and atolls, along with islands proper. And China in particular, has tried to enlarge them.

These enlargements cannot change the legal status of the atolls, reefs or rocks - only islands get an EEZ. But they can still be used to project power - and to bolster a country's claims.

China took control of Fiery Cross Reef in 1988. Even in 2009, it remained a rock.

But by 2015, land had been reclaimed and a harbour and runway built. There are radar and observation towers, and even recreational facilities for troops.

This is a full-on military installation. Crucially, they can host anti-ship and anti-aircraft missile systems too.

Or take Subi Reef, previously just a low tide elevation. But 976 acres have been reclaimed and a runway has somehow been squeezed into the thin circle of land.

No country has been as vigorous as China in digging out new land in the middle of the sea - but others have been fortifying the region too. Take Thitu island, where there's a runway and various buildings.

It is the second-largest naturally formed land feature in the disputed Spratly Islands and has been occupied by the Philippines since 1974. And just recently, the Philippines said they plan to "fortify" the islands they occupy, including Thitu.

China tends to reject the charge of militarising the South China Sea - and likes to point to US actions. In 2021, a Chinese foreign ministry spokesperson said: "It is the United States which has actually militarised the area and threatened freedom of navigation… the country, over 8,300 miles from the South China Sea, has built several military bases with offensive weapons around the area, and frequently sent aircraft carriers and strategic bombers throughout the year." The US does not claim any part of the South China Sea but it does frequently send warships on freedom of navigation operations.

And it owns or has access to bases that surround the region - and China. Following Sky News witnessing a Chinese ship firing water cannon at a Philippine coastguard vessel that was on a resupply mission to Scarborough Shoal, we approached the Chinese embassy in London for comment.

In reply, it referred to remarks by a spokesperson for the Chinese ministry of foreign affairs who said: "Huangyan Dao has always been China's territory. "The Philippine coastguard vessel and official vessel entered waters of Huangyan Dao without Chinese permission, which seriously infringed on China's sovereignty.

"The China coastguard took necessary measures in accordance with the law, and the way it handled the situation was professional, proper and lawful. China urges the Philippines to stop making infringement and provocations at once and not to challenge China's resolve to defend our sovereignty.".