How the deep-sea cables that power the world are made

Observer

14-07-2025

On a recent spring day, a chunky, 560-foot-long ship with a bulbous nose and a stern bristling with heavy equipment sat at the end of a slender quarter-mile pier off the forested coast of Finland.
The vessel, called the Mona Lisa, which was recently built in Romania for approximately 30 million euros, will serve as a crucial instrument in the expansion and reconfiguration of electric power systems worldwide.
Over a week, about 80 miles of high-voltage cable snaked out from a factory to the boat, where it was coiled into tall stacks on big turntables.
This complex in Pikkala, on the outskirts of Helsinki, the Finnish capital, is one of the few places in the world that can produce conduits with the capacity to link countries and the durability to withstand the rigors of the ocean depths.
Electric power is no longer the humdrum industry it once was. In the next decades, the world is expected to experience increased demand for electricity to feed a variety of needs, from data centers to electric vehicles. The power grid itself is also being modernized and extended to reach new sources of generation and trade energy across borders.
Undersea routes are often the preferred option for sharing power between countries or simply keeping cables out of sight.
For Prysmian, the Milan-based company that operates the factory, demand for these heavy-duty cables is far outrunning what it can supply.
'We are sold out through 2028,' Massimo Battaini, CEO of Prysmian, said in an interview at the plant. He added that orders on the books for these conduits, which can carry up to 2 gigawatts of power, have jumped to around 17 billion euros from 2 billion euros five years ago.
The market is so tight, experts say, because making these high-capacity cables is difficult and time-consuming. To form the cores that conduct electricity, as many as 161 strands of copper or aluminum need to be wrapped together. At the factory, these wires — often three to a cable — are encased in polypropylene insulation, jacketed in plastic and lead, and armored with a metal sheath.
It is more efficient to make the cables if they are hanging vertically, so manufacturing lines run up medieval-looking towers. The Finnish plant, which has about 510 employees, recently added one more than 600 feet high to increase production to around 1,150 miles a year.
The cable industry also faces President Donald Trump's 50% tariff on imported aluminum and a similar levy on copper, which could raise costs for Prysmian.
But the company said it should be well-positioned with a strong manufacturing base in the United States that uses large quantities of locally sourced copper.
The Finnish cable-making operation dates back more than a century, when horses and carts were used to lay cables on frozen lakes. Italian conglomerate Pirelli, which later spun off a unit that is now Prysmian, bought the plant from Nokia in 2000.
'If another company wanted to get into that market, it's quite hard for them to do so without spending years figuring out all the clever manufacturing techniques,' said Tim Green, a professor of electrical power engineering at Imperial College in London.
After loading the cable, the Monna Lisa headed across the Atlantic Ocean to help hook up a wind farm being built off the coast of Virginia.
When laying cable, the powerful boat pulls a plow that, with the aid of water jets, digs a trench as deep as 16 feet and simultaneously buries it, giving some protection against accidents like dragged anchors or deliberate sabotage. Last year, a power cable and communications links connecting Finland to other countries around the Baltic Sea were severed by a ship dragging anchor.
The growing importance of these cables creates 'vulnerabilities' that the company is trying to reduce through means like embedding sensors in the cables for early warning and having a repair ship on standby, said Raul Gil, executive vice president for power transmission at Prysmian.
A remote-controlled vehicle makes sure the cables are following the planned route. Powerful motors and a sophisticated guidance system help the boat hold its position when the weather becomes too rough, preserving the cable and saving time and money.
'This means we don't have to cut the cable,' said Valerio Acquaotta, who manages the fleet, even if that means staying in one place for a couple of days.
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