The Persian Gulf: a shallow sea that moves the world's energy market

The Persian Gulf is a shallow arm of the Indian Ocean wedged between Iran to the north and northeast and the Arabian Peninsula to the south and southwest. It runs roughly 990 km from the mouth of the Shatt al-Arab down to the Strait of Hormuz, and its width swings from about 340 km at its widest down to just 55 km at that strait — the single gap through which its water reaches the Gulf of Oman and, beyond it, the Arabian Sea.

Eight countries share its coastline. Iran is the only state that borders both the northern and eastern shores; Iraq holds the northwestern corner, while Kuwait, Saudi Arabia, Bahrain, Qatar, the UAE, and the northern tip of Oman wrap around the southern and western half.

The Gulf is one of the shallowest marginal seas anywhere in the world. Average depth rarely tops 90 metres, and only near the strait and in a few isolated spots toward the southeast does it pass 110 metres. What's more interesting is how uneven the seafloor is: the deepest channel runs tight against the Iranian coast, while the Arabian side is a wide, shallow shelf that rarely drops past 35 metres.

The islands scattered across the Gulf come from two very different origins: some are salt domes pushed up from deep underground, and others are simply built, layer by layer, from coral debris and the skeletal remains of marine organisms.

Almost no fresh water reaches the Gulf. What little does comes from the Tigris, Euphrates, and Karun rivers via the Shatt al-Arab in the northwest; the Arabian side has essentially no permanent river at all. Combine that with relentless heat and heavy evaporation, and the water loses far more than it gains — pushing salinity to between 37 and 41 parts per thousand, and even higher in the shallow lagoons along the Arabian shore.

Water circulates counter-clockwise and, crucially, vertically: surface water flows in from the Indian Ocean, gets concentrated by intense evaporation, sinks as it cools and thickens, then slips back out toward the open ocean as a deep undercurrent, running beneath the very water that just flowed in.

Today's Gulf, together with the low plain to its northwest now buried under Mesopotamian river sediment, is really what's left of a much larger sedimentary basin that stretched northwest to southeast across huge stretches of geological time. Layer upon layer of limestone, marl, and evaporite deposits built up in that basin alongside rich organic material — the same combination that, over millions of years, became one of the largest oil and gas reserves on Earth.

Before oil was discovered in Iran in 1908, life around the Gulf revolved around fishing, pearl diving, building dhows, date cultivation, and nomadic herding. Natural pearl diving largely collapsed once Japanese cultured pearls reached world markets in the 1930s, but commercial fishing later grew into a real export industry in Kuwait, Qatar, and Bahrain — though dam-building on the rivers upstream has since cut the flow of nutrients reaching the Gulf and hurt fish stocks in the northwest.

What really put the region on the global economic map, though, is something else entirely: the countries around the Gulf hold close to three-fifths of the world's proven oil reserves and roughly a third of its proven natural gas. From World War II onward, the region's share of global oil output has stayed enormous.

That same central role in moving the world's oil and gas is exactly what has turned the Gulf into the stage for some of the region's most serious military tensions. During the Iran-Iraq War (1980–1988), both sides began striking each other's tankers in what became known as the Tanker War — a campaign that hit more than a hundred vessels and eventually drew the United States and several western European navies in to escort shipping through the Gulf.

Sporadic naval standoffs between Iran and the United States kept recurring in the Gulf and the Strait of Hormuz in the decades after. One of the most serious episodes came in early 2026, when rising tension between Iran and a US-Israel coalition escalated into joint military strikes by the two countries against Iran, which responded with missile, drone, and naval action against targets in the Gulf and the strait. Iranian warnings about mining or closing the strait sharply curtailed tanker traffic through the passage for a period — a passage that carries a large share of the world's exported oil and liquefied natural gas.

The video below documents natural gas extraction at the North Field off the coast of Qatar — one of the largest shared gas fields on Earth, sitting right in the Gulf. Subtitles are available in all four languages this article is published in.