You can hear them before you see them, a distant sputter of engines carrying across the seemingly empty waters of the Strait of Hormuz.

Then, out of the haze, they appear. Speedboats – dozens at first, then hundreds – bouncing over the swell, each carrying no more than two or three men, their faces wrapped in scarves against the spray.

For the next two hours, they stream past at a relentless pace, churning across the narrowest stretch of the Strait where our boat bobs on the waves.

The constant traffic offers a clue to a larger truth. Sealing the world’s most important waterway is one thing. Reopening it is quite another.

Although few vessels now pass through a Strait under double blockade, plenty still move across it. Within this swarm, it would be almost impossible to distinguish a trader from a boat laying mines.

The Strait of Hormuz may be only 21 miles across at its narrowest point, but making sense of what crosses it is a challenge. Only gradually does the confusing picture sharpen into focus.

Despite cloudless skies, visibility is poor. The shallowness of the water, the intense heat and the jagged cliffs hemming in the Strait trap moisture, shrouding the sea in an opalescent haze.

North of us, one can just make out the silhouettes of cargo vessels – oil tankers, bulk carriers and smaller ships – lying at anchor in the Gulf, stranded for weeks by the closure of the channel.

To the south, beyond the bend of the Musandam Peninsula, US warships lie in wait to intercept vessels attempting to leave Iranian ports. The Strait itself appears empty until you see the speedboats.

Fears about control of the Strait date back more than a century to when oil replaced coal as the fuel of the global economy. Britain intervened here in the 1950s after Iran nationalised the Anglo-Iranian Oil Company, and again in 1970 to help Oman defeat a Moscow-backed insurgency that threatened to give the Soviet Union a foothold on the southern shore.

Had that happened, some historians argue, Moscow would have had control of the world economy’s jugular vein and might even have won the Cold War.

Yet a full blockade remained theoretical until the US and Israel attacked Iran on Feb 28.

For years, analysts doubted Iran had enough strength to close the waterway. Donald Trump reportedly shared that view, accepting Israeli assessments that Iran was too weak to seal the Strait. After all, the Iranians would not just be taking on Israel but also the most formidable military power.

They were wrong.

Even after its conventional fleet was destroyed, Iran has found it far easier to hold the world economy to ransom than many assumed, largely because modern naval warfare favours the Davids over the Goliaths.

The lesson was already visible in the Black Sea. Ukraine, despite losing much of its navy, drove back Russia’s fleet, sinking a third of its warships and forcing the rest to retreat from the naval base at Sevastopol, in occupied Crimea. The battle for control of the Black Sea was won not by the mighty, but by the nimble, with Ukraine’s so-called “mosquito fleet” humbling its enemy with a trifecta of sea drones, missile strikes and limpet mines.

Iran has applied a similar logic. Since the war began, it has attacked more than 20 commercial vessels in and around the Strait, killing 10 seafarers and injuring many others, using its own version of the mosquito fleet doctrine.

Its navy may be crippled, but fast-attack craft operated by the Islamic Revolutionary Guard Corps (IRGC), armed with rockets, machine guns and short-range missiles, have repeatedly evaded US naval forces. Sea and air drones have also played a role.

The most effective weapon has been the least visible: mines. A saturation-mining campaign has seeded the Strait with an underwater threat that shipping companies and their insurers are unwilling to risk.

It is here that the speedboats play a vital role. Most of those racing past us are innocuous enough. Some are smugglers, others licensed, most operating in the grey zone between.

With petrol in Iran costing less than half the price than in the Gulf, fuel smuggling is lucrative. Boats cross laden with plastic drums, then return with supermarket goods and electronics. Larger vessels carry second-hand cars, though many now stay clear for fear of mines.

For coastal communities outside cities such as Dubai, Abu Dhabi and Muscat, this trade is an economic lifeline.

It also provides perfect cover. IRGC speedboats and fishing dhows blend seamlessly into the traffic, able to drop mines over the stern without specialised equipment. In the haze, spotting them is harder still. Some intelligence estimates suggest Iran used dozens of such vessels to lay as many as 150 mines in a 48-hour period in early April.

“It is hard to see them and harder to stop them,” one Western defence official says. “For the moment, the best solution for clearing the Strait is diplomacy.”

Tehran, however, has little incentive to relinquish its advantage.

Closing the Strait cannot be sustained indefinitely. Gulf states will build pipelines to bypass it. Countermeasures will improve. In time, the US’s counter-blockade will tighten the pressure on Iran’s tottering economy, potentially triggering fresh unrest.

But that means it has even more reason to use its leverage now.

“It can increase the pain it is inflicting for a while longer before the pain it is suffering becomes more difficult to manage,” said Farzan Sabet, a managing researcher at the Geneva Graduate Institute. “But what that means is that in the interim it is feeling quite confident. It knows its greatest leverage is in the next few weeks and months.”

In the meantime, Iran seems to be escalating rather than rowing back.

In recent days, the IRGC has seized commercial vessels, fired on others and reportedly laid additional mines, despite an order from Mr Trump to “shoot and kill” any Iranian boats laying mines with “no hesitation”.

That, the defence official says, is easier said than done: “You have to find them before you can kill them. That’s the problem.”