More than 25 years ago, the BRP Sierra Madre was sent off for one final, secret voyage. In the darkness of night, the Philippine navy ship sailed from Manila Bay into the remote waters of the South China Sea. Then, to the surprise of many, it ran aground, and hasn’t moved since.

“I did it as quietly as I could, so I would not raise any hackles among anybody,” says Vice Adm Eduardo Santos, who was chief of the navy at the time. To him, it was a case of mission accomplished. His plan had been to run the ship on to a small reef known as Second Thomas Shoal, one of the world’s most fiercely contested maritime sites, without China knowing. The move would help the Philippines defend the area for decades to come.

“The first reaction was the Chinese ambassador knocking at my office early in the morning when they heard about it … I said, ‘well, it was supposed to be on the way [to a mission], and it ran aground’,” says Santos. With hindsight, Santos, who is now 80, can smile about it, though he, more than most, is keenly aware of how delicate the issue remains.

If the shoal had been left unoccupied, it would have been lost to Beijing, he says, because the Philippines was already facing a “creeping invasion” by China.

Beijing had already seized Mischief Reef, an atoll just 21 nautical miles away, despite being within the Philippines’ exclusive economic zone (EEZ) – an area that stretches 200 nautical miles from a state’s coast, giving it special rights to build or exploit resources in the area. Second Thomas Shoal is also with the Philippines’ EEZ.

The Sierra Madre’s last journey, in 1999 was a crucial milestone in the wider, long-running dispute over the South China Sea, a region that hosts one of the world’s busiest trade routes, and thought to be rich in oil and gas deposits.

In the years since, the crumbling vessel’s dogged presence at Second Thomas Shoal has enraged Beijing. It has been at the centre of at least eight maritime confrontations in the past 18 months alone, as China has sought to disrupt missions delivering supplies to the shoal. The ship serves as a de-facto military outpost, preventing China from expanding further towards the Philippines’ coast. China claims much of the sea, despite a Hague tribunal ruling finding otherwise.

Tanks loaded, guns ready, but only one engine

In the 1990s, there were few options for the Philippines, says Santos, beyond a cat-and-mouse game of removing structures being put up by China. The exit of US forces from the Philippines in 1992 after a 94-year presence had also left a vacuum, he adds.

The navy’s attention then turned to a ship that was due to be decommissioned: BRP Sierra Madre, a tank-landing vessel built for the US navy in the second world war that passed to the Philippines, a US ally, in 1976.

Months were spent reviving it. “I had to do some repairs, quietly, make sure that the tanks were loaded, ammunition loaded, guns firing and all that,” Santos says. “I needed power for the radar and for the people that would be staying there.”

Only one engine was fitted, rather than the usual two; it wasn’t expected to be doing much future travel. It was sent off in May 1999, with about 20 men on board, says Santos. Only the captain knew details of the mission.

Soldiers would be based on the vessel for three months or so. “In the beginning, it was nice because we fixed the ship. It’s painted, the interior … [There’s] air conditioning. They had the Betamax ,” he says. There was a karaoke machine, and the helipad was used for basketball. Troops were already used to the loneliness of being stationed far from home and, in a region known for typhoons and intense storms, the Sierra Madre was preferable to a small island.

“In their spare time, they would be fishing and then drying up the fish and then selling the dried fish when they would go home, so they had extra income,” says Santos.

However, after decades of being battered by strong winds, and weathered by sea spray, the ship is in a dilapidated state, with some fearing it is on the brink of collapse – a prospect that could leave the shoal vulnerable to China once again. Beijing has repeatedly blocked efforts to resupply the vessel, particularly those it believed to be carrying building materials, though it says it acts professionally and with restraint.

China claims the Sierra Madre’s presence is illegal and has demanded it be towed away.

“I’m told that they were able to do some minor alterations so that the ceiling of the ship would not [leak], so during the rain, they would not get wet,” he said. “They were able to do some things to be able to sleep on a dry space.”

Even walking across the main deck was, at times, hazardous according to one report, which said visitors would need to walk across a plank because it had deteriorated so heavily.

Santos said he was saddened by the state of the ship. “There’s always a possibility that it may not really last longer, it may not really last for another five years maybe,” says Santos, who added he wasn’t sure of the latest details on the state of the ship.

‘We cannot fight them by force of arms’

China, he adds, wants the whole of the South China Sea and East China Sea. “There’s almost US$3tn worth of goods flowing through that area [every year] … There’s always an opportunity to make money out of that,” he says. “Of course, there’s also oil in that area. [China] knows that because they did joint surveys with us.”

Santos welcomed the government’s broader efforts to publicise incidents where Chinese vessels act aggressively at sea, and to deepen defence ties with allies and partners.

A recent survey found that 73% of Filipinos want both military and diplomatic action to defend the West Philippine Sea, the name the country uses to describe its exclusive economic zone. It’s easy to say in a survey that you want to fight, says Santos. “For us in the armed forces, no, we don’t like to fight.”

“We cannot fight them by force of arms. [So] then we should use the law. I would think that the Philippines is now moving to file another case with the permanent court of arbitration. We already got the favourable decision of 2016 [in a tribunal at The Hague]. We could do that now,” Santos says.

Many have suggested the Philippines should try to ground another ship, in the same style of the Sierra Madre, to protect other locations within the Philippines’ EEZ. Given China’s now-extensive surveillance capabilities, he doubts this would be possible: “They know already … When they saw the same thing happen in another island, they immediately demanded that it be removed,” says Santos.