The war between the U.S. and Iran has generated a new phenomenon, tankers carrying millions of barrels of petroleum in secret through the contested Strait of Hormuz. These dark ships are deliberately disappearing from electronic tracking systems as they move through one of the world’s most dangerous waterways.

Such techniques were once associated primarily with Iran’s sanctions-evading shadow fleet, but they have increasingly been adopted by conventional Gulf exporters trying to move oil despite attacks, blockades, and military threats.

Data intelligence company, Kpler estimated that ship-to-ship transfers and clandestine movements helped move roughly 1.9 million barrels per day through Hormuz beginning in April, with other estimates reaching between 2.1 million and 2.9 million barrels per day during periods of heavier activity.

The secret transporting of oil through the Strait of Hormuz is likely one reason global oil prices have been kept down despite the disruption to merchant traffic due to the war between the U.S. and Iran. Normally, ships continuously broadcast their identity and position through the Automatic Identification System, or AIS. During a dark transit, tankers switch off their transponders, sometimes dim their lights, and cross the strait without appearing on ordinary commercial tracking systems.

Reuters reported that Vortexa estimated about 65 percent of laden tankers leaving Hormuz in May traveled in dark mode. More recently, Lloyd’s List reported that all identified non-Iranian tanker passages during one week in early August were conducted as dark transits, demonstrating that the practice has become a significant feature of wartime shipping rather than simply an Iranian sanctions-evasion technique.

Because the ships involved in the secret transport of oil conceal their movements, it is difficult to determine exactly how much oil is being transported that way. But the ships involved are no longer exclusively Iranian. Iran pioneered many of these methods to evade American sanctions, including disabling AIS systems, disguising ownership, and transferring oil between tankers at sea.

But Gulf exporters have increasingly adopted similar techniques. A Reuters investigation found that beginning in May, at least 116 ships participated in an elaborate system in which smaller tankers traveled through Hormuz and transferred crude to larger vessels waiting near Oman and the United Arab Emirates. Reuters reported that multiple sources described U.S. military surveillance and protection of these operations, although a U.S. defense official denied that Central Command forces were participating in offshore oil transfers.

Turning off electronic positioning systems and lighting can be dangerous for ships involved by increasing their chances of running into other ships, especially at night. Concealment would also make it difficult for rescuers to locate a stranded or disabled ship, which increases the risk of operating in the dark.

For world energy markets, dark shipping is both a safety valve and a new source of uncertainty. Before the war, roughly 20 million barrels per day moved through Hormuz; the International Energy Agency calculated that flows averaged only 2.7 million barrels per day during March, April, and May, helping produce what it called the largest oil-supply disruption in history.

Dark tankers put additional petroleum into world markets, limiting some upward pressure on prices. But they also make the true global oil supply increasingly difficult to calculate. Traders, governments, and refiners may no longer know precisely how much crude is available. The result is a strange new energy market in which millions of barrels can be physically moving toward consumers while temporarily disappearing from the digital systems the world normally relies upon to see them.