The South Pacific nation of Tonga was all but cut off from the world after an undersea communications cable was severed following the massive eruption of the Hunga Tonga-Hunga Ha'apai volcano on January 15. The blackout caused major disruption to aid efforts following the disaster.

Limited satellite connectivity has provided some relief but communication remains difficult as some outlying islands are still cut off.

Tonga signed a 15-year deal to secure satellite connectivity following an earlier cable break in 2019 from a ship's anchor. But the use of satellite phones has been affected by the volcanic ash blanketing the country. Some people have reported they can only dial out - and not receive calls.

A specialist ship is en route to the area to restore the cable. The ship left Port Moresby, Papua New Guinea, about 4,700 km (2,900 miles) away, on January 20 and is expected to reach the area on January 30. The operation is expected to take a few weeks.

The 827 km (514 miles) cable from Fiji to Tonga is one of 436 active undersea cables that connect the globe.

It was not until five days after the volcanic eruption that the country’s major telecom operators could establish a connection to the global internet via satellite, according to network monitoring company Kentik.

However, the capacity of satellite-based internet is greatly constrained, Doug Madory, a network analyst at Kentik, said. “Satellite service in the Pacific is expensive because of the vast coverage area and relatively small population,” he added.

Tonga’s cable was laid in 2013 at a cost of $15 million. The World Bank and Asian Development Bank extended grants to help it overcome reliance on expensive satellite-based communication.

Undersea communication cables can vary in size and structure but are roughly the diameter of a garden hose. Although relatively thin, these cables have multiple layers protecting the fibre optic cable at the centre. Despite layers of protection and avoiding accident prone areas, on average, there are 100 cable faults recorded every year according to Telegeograph, a telecommunications research firm.

Structure of a subsea cable

Most cable damage occurs due to ship anchors or fishing trawlers and occasionally environmental factors such as earthquakes.

Faults are common and typically most traffic would be rerouted to another cable. However, in Tonga’s case, there is only one cable connecting the country. It also sits on the Pacific Ring of Fire, prone to earthquakes and volcanic eruptions, adding an additional risk.

According to Jonathan Brewer, a telecommunications engineer at Telco2 limited, it's better to have multiple cables or multiple cable landing stations to manage the risk of disruption. However, this can be expensive. Pacific islands like Guam and French Polynesia have significant support from the U.S. and French governments enabling them to have such extensive infrastructure.

The repair ship, CS Reliance, is one of six identical ones owned by SubCom, an undersea cable laying company. It is based in New Caledonia, a French overseas territory west of Tonga, and is one of the largest vessels of its kind.

As of June 2021, there are 59 operational cable laying and maintenance vessels that are periodically deputised to either lay cables or fix faults according to the International Cable Protection Committee.

A light pulse sent through a fibre optic cable would usually travel to the other end. However, in a broken fibre, the pulse bounces back and engineers are able to measure the time it takes to return in order to locate the break.

“Locating the cable can be a challenge if it's buried or if it's moved far from its original route. If the volcano blast or tsunami shifted or collapsed a seamount on top of the cable, it could be very difficult to locate or retrieve,” said Brewer.

A deep sea hook is lowered which cuts the cable into two. One end of the cable is held by a buoy and the other is brought onto the deck. The cable is spliced and repaired onboard with a new joint assembly.

The fibre optic cable isn’t easy to fix. A technician splices the glass fibres and uses glue to attach the new section of the cable. This fibre optic splicing can take up to 16 hours and is the most crucial aspect of the repair work.

After the splicing is completed, the cables are attached to each other and are wrapped in multiple protective layers so they withstand pressure and environmental damage.

The cables are then joined onboard the vessel and gently lowered back down to the sea floor in a hairpin pattern.

Also subject to the conditions to the seabed, the cable-laying vessel may tow a sea plow across the ocean floor to bury the cable.

A remotely operated vehicle (ROV) may descend to the seabed to inspect and help bury the cable, although they can only work to a certain depth. In the case of the Reliance, the vehicle can descend up to 2,500 meters.

Most of these cables have an approximate life span of about 25 years and are usually retired and replaced with newer ones. Some are salvaged for their raw materials that can be repurposed.

Although telephone links between Tonga and the wider world have begun to be reconnected, restoring full internet connectivity is likely to take a month or more according to the owner of the archipelago's sole subsea communications cable.