When a tanker loaded with Russian oil slows in a shipping channel, a waiting diver swims from a mini-submarine and attaches a hull-clinging limpet mine. Days later, when the tanker is plowing the waves of another sea, usually the Mediterranean, a timed charge goes off.

It sounds like the plot of a World War II frogman movie. But it is the leading scenario spun by British security analysts to explain why six tankers in six months have suffered explosions after taking on cargo at Russian ports. Magnetic limpet mines are blamed for four of the attacks. Bombs smuggled on board by saboteurs are blamed for the other two.

In times of peace, explosions on ships in the Mediterranean are rare. Now, jitters spread through marine insurers, the risk-phobic companies charged with covering $50 million tankers. Russia’s government now mandates that shippers use cameras and remote sensing devices to check hulls before entering Russian ports.

“The threat is looking nasty,” Lloyd’s List’s chief editor, Richard Meade, wrote Monday. “In the year to date, five vessels have been hit with what we are now pretty certain are magnetically attached, timer-activated explosive devices.”

The editor of the London-based shipping news added: “Their use requires a high degree of planning and technical sophistication, and suggests unnamed state actors.” 

No state or group claims the attacks. In all cases, the common denominator is a recent stop at a Russian port of call. Two tankers carried oil of Kazakh origin. The goal seems to be to raise the risk — and the price — of trade with Russia.

In the latest attack, on Sunday, two bombs hit Eco Wizard, a brand new liquefied petroleum gas tanker anchored at Ust-Luga, an industrial port 100 miles west of St. Petersburg. Water flooded the engine room, nearly causing the $125 million, Greek-owned vessel to sink.

The engine room of another Greek-owned tanker, Vilamoura, was also damaged by an “unidentified explosive device,” the owner, TMS Tankers Ltd., said in a statement Sunday from Piraeus. Ukraine’s military intelligence released a statement charging that the oil tanker, which carried Kazakh oil loads from two Russian ports this spring, is part of Russia’s so-called shadow fleet. 

The Greek owners say they obey European rules and sanction laws. Vilamoura is the third tanker owned by the company to be bombed this year. Each ship was hit by multiple explosions.

Vilamoura was hit shortly after leaving Zuwetina, eastern Libya, laden with 1 million barrels of crude oil. Photos show that shrapnel tore through three decks of the vessel. So far, no one has been killed or injured in the cargo ship bombings.

After the Vilamoura incident, “shipowners have set a new norm: standard deployment of divers and remotely operated vehicles to scan for mines or sabotage in hulls on a regular basis,” Charles Mitchell writes in Modern Engineering Marvels. The article is titled: “Tanker Explosions, Russian Port Links, and the Engineering Race to Secure Maritime Oil Flows.”

Four of the ships hit by explosions also made port calls in Libya, a country riven by civil war. Some security analysts speculate that saboteurs could have planted explosives during cargo stops in Libya or on supply stops in Malta’s outer harbor.

“It is highly unlikely that naval mines would be placed whilst the vessel was alongside in port, given the risks involved, but there are opportunities en route to/from ports where vessels have slowed down sufficiently for divers to attach limpets,” a British risk and insurance analytical company, Ambrey Analytics, writes in a recent report. The explosions were probably caused by mines with delayed fuses, sophisticated operations that imply state backing, the company says.

Similar to the September 2022 explosions that cut the Nord Stream gas pipelines between Russia and Germany, potential financial liabilities are likely to keep any government from taking credit.