The Strait of Hormuz

Find the narrowest point between Iran and the Arabian Peninsula. Here the sea pinches to thirty-nine kilometres, and in 2024 about twenty million barrels of oil a day squeezed through it.

That was about a fifth of all the oil the world used. Since the war that began with American and Israeli strikes on Iran at the end of February 2026, ships here have been warned off, mined, seized and attacked.

Why does so much oil sit behind one gap, who has tried to hold it, and why is closing it felt at every fuel pump on Earth?

The Gulf is a shallow sea about a thousand kilometres long. Iran, Iraq, Kuwait, Saudi Arabia, Bahrain, Qatar, the United Arab Emirates and Oman all touch its waters.

Iran calls it the Persian Gulf; Arab states call it the Arabian Gulf.

It has only one door to the open ocean: the Strait of Hormuz. On the north side lie Iran's coast and its islands, Hormuz, Qeshm and Larak. On the south, Oman's Musandam peninsula pushes north like a rocky fist.

At the narrowest point, between Iran's Larak island and Oman's Great Quoin island, the gap is about thirty-nine kilometres. That sounds roomy. For a fully loaded supertanker, it is not.

Big ships keep to a fixed route called a traffic separation scheme: one lane in, one lane out, each two nautical miles wide, about three point seven kilometres, with a buffer of the same width between them.

Those lanes lie in Omani waters. Every tanker leaving the Gulf funnels into the one outbound lane, so a single mine, or a single burning ship, can stop the whole queue.

Why is so much oil waiting behind this door? Go back to the age of the dinosaurs, when a warm ocean called the Tethys covered this region.

Its sunlit water teemed with plankton. When they died they sank into the mud, and layer after layer of sediment buried them.

Buried deep, heated and squeezed, that organic mud slowly cooked into oil and gas. Rock with just two percent organic matter counts as a good source; some layers around the Gulf hold up to thirteen.

That is good source rock, but a Norwegian geologist points out it is no better than elsewhere. What made the Gulf special is what happened next.

The Arabian plate has been pushing north into Eurasia for tens of millions of years, and it still is. The collision crumpled the rock into the Zagros Mountains of Iran.

Seen from above, the Zagros are long parallel ripples, and those ripples run on under the Gulf and into Arabia as broad domes.

Each upward arch is an anticline. Oil is lighter than water, so it creeps up through porous rock until it is caught beneath the crest, often sealed under a lid of salt.

The result: roughly half of the world's conventional oil reserves, and forty percent of its gas, lie beneath about three percent of Earth's land.

Long before oil, the strait was valuable for the same reason it is now: everything moving between the Gulf and the Indian Ocean has to pass through it.

From around the eleventh century, a trading kingdom based on the small island of Hormuz charged a toll on the ships that came through.

The island was small. Its wealth came from position alone: it sat on the door.

In 1507 a Portuguese fleet under Afonso de Albuquerque attacked Hormuz, forced a treaty and began a fort. The first hold did not last, but by 1515 Portugal controlled the island.

Portugal kept the toll and tightened it with a licence called the cartaz, without which ships risked seizure. A Portuguese chronicler put the customs at around two hundred thousand cruzados a year.

In 1622 a combined Persian and English force took the island. Trade moved to the mainland port of Bandar Abbas, and the kingdom faded. The strait kept its name.

For almost three centuries the strait was one trade route among many. Then, on the twenty-sixth of May 1908, drillers struck oil at Masjed Soleyman, in the Zagros foothills of Persia.

It was the region's first modern oil discovery, and it sat right on the folds we just watched form.

Within thirty years Iraq, Bahrain, Kuwait and Saudi Arabia had struck oil too.

For most of the fields around the Gulf, the cheapest way to the world's markets was by sea, and that sea has one exit.

The strait's modern danger first played out in the 1980s. Iran and Iraq went to war in September 1980, and each side tried to choke the other's oil income.

Iraq began attacking shipping in 1981. In 1984 it struck Iran's main oil terminal at Kharg Island, and Iran answered by attacking tankers serving Kuwait and other Gulf Arab states.

By the end, one widely cited count lists about four hundred and fifty attacks on ships: some two hundred and eighty by Iraq and one hundred and seventy by Iran. Well over a hundred sailors were killed.

In 1987 Kuwaiti tankers were re-registered under the American flag, and the US Navy began escorting them, in what is often described as the largest naval convoy operation since the Second World War.

On the first day, in July 1987, the reflagged tanker Bridgeton struck a mine. Nobody was hurt, but mines became the weapon to fear.

In April 1988 a mine badly damaged the American frigate Samuel B. Roberts, and the United States struck Iranian targets in Operation Praying Mantis.

In July a US cruiser shot down Iran Air Flight 655, killing all two hundred and ninety people aboard. The US said its crew mistook the airliner for a fighter; Iran says it was shot down knowingly. The tanker attacks ended weeks later.

Of all the world's oil chokepoints, only the Strait of Malacca carries more.

Much of Hormuz's oil goes on through Malacca too. In 2024, eighty-four percent of the crude through Hormuz went to Asia, and China, India, Japan and South Korea took sixty-nine percent.

Most of it rides in very large crude carriers, ships about three hundred and eighty metres long that each hold about two million barrels. Twenty million barrels a day is the cargo of about ten of them, every day.

It is not only oil. About a fifth of the world's liquefied natural gas trade, mostly from Qatar, also leaves through the strait, and the pipelines that go around it carry crude oil, not gas.

So what happens when the valve closes? There are two big pipelines that go around it. Saudi Arabia's East–West line carries crude across the desert to the Red Sea, with about five million barrels a day of capacity.

The Emirates' line runs to Fujairah, outside the strait, with about one point eight million. But both already carry oil in normal times.

The US Energy Information Administration estimated that only about two point six million barrels a day of spare capacity could be switched over. Twenty million in, two point six around. The rest has to wait.

That gap is why fighting in this strait is felt in Tokyo, Mumbai and Bangkok. Thailand, which in early 2026 bought more than half of its imported crude from the Middle East, halted its exports of crude and refined fuel within days of the war starting.

Who owns this water? Iran and Oman each claim twelve nautical miles of sea. Twelve plus twelve is more than the twenty-one-mile gap, so none of it is open sea.

The 1982 UN Convention on the Law of the Sea, UNCLOS, gives ships a right of transit passage through straits like this one, continuous and quick, and says that right may not be suspended.

But neither Iran nor the United States has ratified UNCLOS. Iran, like Oman, recognises only innocent passage, which a coastal state may suspend for its security, and Iran requires warships to ask permission first.

The United States rejects both countries' claims and contests them at sea. The argument has never been settled, and in a crisis each side reads the law its own way.

Since March 2026 the strait has been closed, half open and closed again. Iran's Revolutionary Guard, the IRGC, says it has full control, and that tankers which ignore its rules will be struck.

The United States says Iran has lost control and that oil flows have largely recovered. Ships and their crews sail between those two claims.

The geology will not change. The oil sits where the Tethys left it, behind the same narrow door. What changes is who stands at the door, and what the rest of the world will pay to keep it open.