They come out of the darkness in the waters off the coast of East Africa, zooming up in speedboats to the sides of massive cargo ships, armed with grappling hooks, AK-47s and rocket-propelled grenades.

Quickly, these modern-day pirates climb aboard their prey: cargo ships that contain food, machine parts and, most recently, oil or enough weaponry to supply a small army. Most of the time they meet no opposition — only frightened, unarmed crews who find themselves prisoners and held for ransoms that have exceeded $1 million.

Based in Somalia, these pirates are only a little like the images of the daring, swashbuckling thieves who have gallivanted through Hollywood movies or adventure stories that have been passed on for generations.

These pirates typically use the Global Positioning System to coordinate attacks along major shipping corridors in the Indian Ocean and Gulf of Aden. A report last month by Chatham House, a British think tank, said that once aboard, Somali pirates typically are focused on demanding a ransom from the ship’s operators and chewing khat, a narcotic leaf that is a stimulant, that they bring with them.

Piracy off Somalia’s coast has long been a symbol of that African nation’s instability. Now attacks on shipping are soaring and becoming more brazen, heightening concerns about the safety of shipping from oil-rich areas in Africa and the Middle East at a time of global economic instability.

The potential for Somali renegades to send tremors through the world’s economy was clear Saturday, when pirates captured their biggest prize to date: the Sirius Star, a Saudi supertanker brimming with 2 million barrels of oil (estimated value: $100 million).

The Times of London reported Wednesday the Saudi government had confirmed that the ship’s owner — Vela International Marine — was negotiating a possible ransom with pirates who boarded the oil tanker more than 450 nautical miles from the Kenyan port of Mombasa.

The pirates’ raid of the Sirius Star — and hijackings Tuesday in the Gulf of Aden of a Thai ship with 16 crewmembers and an Iranian cargo vessel with a crew of 25 — are signs that attacks by loosely organized bands of Somali pirates are “a criminal enterprise which has gone completely out of control,” says Capt. Pottengal Mukundan, director of the International Maritime Bureau, which tracks piracy.

U.S. and British analysts say the series of raids underscore worries that terrorists could dive into the same lawless seas off East Africa, capture booty to finance their operations or mount a spectacular attack with a seized ship.

“There is serious concern that terrorists see piracy as an opportunity for themselves,” says Roger Middleton, an expert on piracy at Chatham House. “It can provide the means to generate enormous amounts of money, or to capture a boat with the more disturbing prospect of a huge oil tanker as a floating bomb.”

In March, the Pentagon confirmed that U.S. forces attacked a suspected al-Qaeda terrorist in Somalia.

Pirates already are driving up the cost of shipping and insurance. Some shipping lines have begun avoiding the shipping corridors near Somalia and their shortcut to the West through the Red Sea and Suez Canal, which can add between five and 10 days to a trip from Asia to Europe, says David Ellis, president of Odfjell USA, a Norwegian-owned shipping company. Each extra day at sea, he says, costs about $30,000.

Environmental catastrophe looms if a supertanker is punctured during an attack or purposely sunk, Middleton says.

The Bush administration is trying to coordinate efforts to stop the pirates, although military officials say they can’t stop all pirates because there are too many ships in a huge area to protect.