Persian Gulf

• 1. A Shallow Sea

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  The Persian Gulf stretches 989 kilometres from end to end, yet its deepest point reaches only 90 metres. Its average depth is about 50 metres, so ships cross a sea that is shallow almost everywhere. Geographers call it a mediterranean sea, meaning a body of water largely enclosed by land. It sits in West Asia between the Arabian Peninsula and Iran, which was once known as Persia.

  People on its southern shores often call it the Arabian Gulf, and the argument over that single word has run for decades. Behind the argument lies a basin that was once dry ground, a sea that must be refilled through one narrow gap, and a coastline shared by eight countries.

  This documentary asks how a drowned plain became a cradle for some of the oldest seafaring evidence on Earth. It asks why empires from Greece to Portugal to Britain fought over its ports. It asks what still lives in its warm, crowded waters, and why the rest of the world pays such close attention to what moves across them.

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• 2. The Drowned Plain

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  About 125 metres of sea level separate the modern Gulf from the world at the Last Glacial Maximum. At that low point the water stood roughly that far below present levels. During most of the Last Glacial Period, which ran from about 115,000 to 11,700 years ago, the shallow floor lay exposed. The basin was a flat floodplain where a number of rivers converged.

  This lost landscape may have been a refuge for early humans when the climate swung into long spells of extreme dryness. Then the ice melted. Between roughly 12,000 and 6,000 years ago, during the early Holocene, rising seas filled the basin and created the marine Gulf. Researchers suggest that the flooding may have helped stimulate Neolithic farming cultures in the neighbouring Middle East.

  The basin itself is far older than the sea that fills it. It dates to the Cenozoic era and is related to the Arabian plate sliding beneath the Zagros Mountains.

  Sumer, the world's oldest known civilization, grew up along the Gulf and across southern Mesopotamia. At a site called H3 in Kuwait, archaeologists found the oldest evidence anywhere for seagoing vessels, dating to the mid-sixth millennium BC. In that period the Gulf belonged to a wide trade network linking the Ubaid settlements of Mesopotamia with communities along its whole coast.

  Those first sailors worked a sea that has to be refilled from the Indian Ocean through one narrow gap.

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• 3. The Salt Engine

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  The Gulf loses 416 cubic kilometres of water every year once its rivers and rainfall are counted against evaporation. Rivers from Iran and Iraq deliver an estimated 2,000 cubic metres per second. Rain over Qeshm Island adds only around 180 millimetres a year. The sun does the rest, because the evaporation rate here is high.

  The missing water arrives through the Strait of Hormuz, which links the Gulf to the Gulf of Oman and is about 56 kilometres wide at its narrowest. Water inside the Gulf grows saltier and therefore heavier, so it leaves along the bottom of the strait. Lighter, less salty ocean water slides in across the top. One study put evaporation at 1.84 metres a year and rainfall at only 0.08 metres.

  At the other end, the Shatt al-Arab delta marks the western limit of this sea of some 251,000 square kilometres. The delta carries the waters of the Euphrates and the Tigris, and Iranians call it the Arvand Rud. Modellers who simulate these flows in three dimensions typically work with cells about 3 kilometres across.

  The International Hydrographic Organization closes the southern boundary with a line from Ràs Limah, at 25°57'N on the Arabian coast, to Ràs al Kuh, at 25°48'N on the Iranian coast.

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• 4. Many Names

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  The Assyrians called this water the Bitter Sea. In 550 BC the Achaemenid dynasty founded the first ancient empire in Persis, also called Pars or modern Fars, in the southwest of the Iranian plateau. Greek writers then named the water bordering that province the Persian Gulf. In his book Indikê, around 300 BC, Nearchus uses the phrase "Persikon kolpos" several times for the same sea. Between 550 and 330 BC, texts frequently speak of the Persian Sea.

  Darius the Great left a fifth-century BC inscription about a canal he ordered dug from the Nile in Egypt. The canal ran toward "the sea that begins in Persia," and when it was finished, ships "went from Egypt through this canal to Persia, as I had intended."

  Under the Sasanian Empire, which lasted from 224 to 651 AD, the Gulf was called Pūdīg. That name descends from Pūitika, which appears in the Bundahishn.

  The modern dispute is much younger. Arab governments call the sea the Arabian Gulf or simply the Gulf, while the International Hydrographic Organization uses the name Gulf of Iran. The Arabian Gulf label first appeared among some Arab states in the 1950s and 1960s, when Arab nationalism was at its height. Rivalry between Iran and certain Arab states has made it the usual term across most of the Arab world since then.

  Britannica notes that outside the Arab states, the name Arabian Gulf has not won wide recognition.

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• 5. Eight Shores

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  Counted clockwise from the north, the Gulf's coastline belongs to Iran, Oman's Musandam exclave, the United Arab Emirates, Saudi Arabia, Qatar, Bahrain, Kuwait and Iraq. Qatar sits on a peninsula off the Saudi coast, and Bahrain is an island nation. Iran holds most of the northern shore and Saudi Arabia most of the southern. Together the eight coastlines total 5,117 kilometres.

  Qeshm, which belongs to Iran and lies in the Strait of Hormuz, is the largest island in the Gulf. Iran also administers Greater Tunb, Lesser Tunb and Kish. Kuwait administers Bubiyan, Saudi Arabia administers Tarout, and the United Arab Emirates administers Dalma. Some of these small islands are the subject of territorial disputes between neighbouring states.

  Man-made islands have joined the natural ones in recent years. The World Islands in Dubai and The Pearl Island in Doha were built as tourist attractions. Older islands carry a different kind of history, since Portuguese and British powers used them for trade or acquired them for their empires.

  Today many major cities of the Middle East stand along these shores. The Gulf's strategic position has long made it an ideal place for settlement, which is why so many powers wanted a share of it.

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• 6. Empires at Sea

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  At the end of the fourth millennium BC, the Dilmun civilization dominated the southern Gulf. For a long time the most important settlement on that coast was Gerrha. In the second century AD, the Lakhum tribe from what is now Yemen moved north and founded the Lakhmid Kingdom along the southern shore. Shapur II of Sasanian Persia later invaded and defeated the Lakhmids, then advanced into Arabia.

  From 625 BC to 226 AD, a succession of Persian empires held the northern side: the Median, Achaemenid, Seleucid and Parthian. Under Darius the Great, Persian ships reached the Gulf. Vessels of 100 to 200 capacity patrolled the Shatt al-Arab, the Tigris, the Nile and the Sind waterway in India. Achaemenid naval bases stood along the Shatt al-Arab and in Bahrain, Oman and Yemen. The fleet also opened trade with India by way of the Gulf.

  Siraf, a Sasanian port on the northern shore in today's Bushehr province, shows how far that trade reached. It first made contact with the Far East in 185 AD and had a thriving trade with China by the fourth century. The Gulf and the Silk Road were the great trade routes of the Sasanian Empire, and in the 7th century the Sasanians ruled the whole Gulf.

  Portuguese influence lasted 250 years from the early 16th century. Afonso de Albuquerque captured the strategic kingdom of Hormuz after Vasco da Gama's voyages. In 1521 a force under Antonio Correia invaded Bahrain to seize the wealth of its pearl industry. On the 29th of April 1602, the Safavid emperor Shāh Abbās expelled the Portuguese from Bahrain. Iran commemorates that date as Persian Gulf National Day.

  In 1615 Abbās took Bandar Abbās and named it after himself. With help from the British fleet, he took the island of Hormuz in 1622, and much of the trade moved to the new town. Portuguese, Dutch, French and British merchants all received special privileges. The Ottomans returned to Eastern Arabia in 1871, when pressure from Midhat Pasha, governor of the Vilayet of Baghdad, led the Al Thani tribe to submit peacefully. The need for troops elsewhere forced them out again at the start of World War I.

  In World War II, after the Anglo-Soviet invasion of Iran, Britain used the Gulf as the entry point to the "Persian Corridor." Supplies for the Soviet Union then travelled on the Trans-Iranian Railway.

  Piracy troubled the Gulf until the 19th century, with the Al Qasimi tribe responsible for many of the best-known cases. The British mounted the Persian Gulf campaign of 1819 and signed the General Maritime Treaty of 1820 with the Sheikhs of what was then called the Pirate Coast. From 1763 until 1971, Britain held varying degrees of political control through the British Residency of the Persian Gulf. Its territory included the Trucial States, the original name of the United Arab Emirates.

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• 7. Energy and Conflict

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  The Safaniya Oil Field, the world's largest offshore oilfield, lies beneath the Gulf. Qatar and Iran share another giant deposit across the territorial median line. The Qatari sector is called the North Field and the Iranian sector the South Pars Field. Using this gas, Qatar built a substantial liquefied natural gas and petrochemical industry.

  In 2002 the Gulf nations of Bahrain, Iran, Iraq, Kuwait, Qatar, Saudi Arabia and the UAE produced about 25% of the world's oil. They held nearly two-thirds of the world's crude oil reserves and about 35% of its natural gas reserves. The Gulf and its coastal areas are the world's largest single source of petroleum. In 2026 about one fifth of global LNG exports originates there, along with liquefied petroleum gas, diesel and jet fuel.

  Fertilizer is a second export that rides the same sea lanes. In the 2020s, Iran, Qatar, Saudi Arabia and Oman have supplied roughly 30 to 35 percent of global urea exports and around 20 to 30 percent of ammonia exports. Up to 30% of all internationally traded fertilizer normally passes through the Strait of Hormuz.

  Iraq's access to the sea is narrow and easy to blockade. It consists of the marshy Shatt al-Arab delta, where Iran holds the east bank.

  The Iran-Iraq War of 1980-88 turned the Gulf into a battlefield, with each side attacking the other's oil tankers. The 1991 Gulf War took its name from the sea after Iraq invaded Kuwait. It was a conflict fought largely in the air and on land. The United States' role in the region grew through the second half of the 20th century.

  On the 3rd of July 1988, the U.S. military shot down Iran Air Flight 655 over the Gulf. It had mistaken the Airbus A300 for an Iranian F-14 Tomcat, and all 290 people on board died. In 2018 the United Kingdom opened a permanent military base in the Gulf, its first since withdrawing from East of Suez in 1971. It is also developing a support facility in Oman.

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• 8. Fragile Waters

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  More than 700 species of fish live in the Gulf, most of them native, and over 80% of them are tied to reefs. Most of those reefs are rocky, and coral reefs are fewer here than in the Red Sea. Silt carried by the great rivers blocks the strong light that reef-building corals need. The same rivers cause big swings in temperature and salinity that corals tolerate poorly. Even so, coral reefs exist along the coast of every Gulf country.

  Coral numbers have fallen sharply in recent years, partly from global warming but mostly from dumping. Construction waste such as tires, cement and chemical by-products reaches the water. It also creates traps in which marine life becomes stuck and dies. Most pollution in the Gulf comes from ships, and pollution from land is the second most common source.

  Whales were once abundant here before commercial hunting wiped them out. Illegal mass hunts by the Soviet Union and Japan in the 1960s and 1970s reduced them further. Today dolphins and finless porpoises are the most common marine mammals. Bryde's whales, the critically endangered Arabian humpback whale, Omura's whale, minke whales and orcas still enter the Gulf. A 2017 finding showed that the Gulf, together with Abu Dhabi, holds the world's largest population of Indo-Pacific humpbacked dolphins. Dolphins in the northern waters off Iran face entrapment in purse seine fishing nets and pollution from oil, sewage and industrial runoff.

  Around 7,500 dugongs live in the waters off Qatar, Bahrain, the UAE and Saudi Arabia. That makes the region their second most important habitat after Australia. Their seagrass diet has been hurt by coastal development, artificial islands and oil spilled during the Persian Gulf war. Uncontrolled hunting has also taken a toll, and nobody knows how many remain or whether their numbers are rising.

  Mangroves of the genera Avicennia and Rhizophora line the shallows and need tidal flow and a blend of fresh and salt water. They shelter young crabs, small fish and insects, which in turn feed marine birds. Artificial island construction is destroying or covering much of this habitat.

  Birds feel the same pressure. A 2006 estimate counted only three viable nesting sites for the kalbaensis subspecies of the collared kingfisher. One lay 80 miles from Dubai and two smaller ones in Oman. A UN plan to protect mangroves as a reserve was ignored by the emirate of Sharjah, which allowed a channel to be dredged through the wetland.

  A 2009 study of ten Iranian islands recorded more than 100,000 breeding pairs of waterbirds from 11 species. They included the bridled tern, the lesser crested tern and the crab plover. Researchers have proposed that swings in these seabird populations could serve as an early signal of the whole region's ecological health.

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