How Submarine Cables Connect the Internet

When you load a website hosted on another continent, your data travels through a cable lying on the ocean floor. Undersea fiber optic cables are one of the most important — and least visible — pieces of infrastructure keeping the global internet alive. Over 400 active submarine cable systems span more than 1.3 million kilometers, carrying more than 95% of all international internet traffic.

What Submarine Cables Are

A submarine cable is a bundle of optical fibers protected by several layers of shielding and armor, designed to survive the crushing pressure, corrosive saltwater, and physical hazards of the deep ocean. Each individual fiber strand is thinner than a human hair, yet a modern cable system can carry 200+ terabits per second across thousands of kilometers.

Despite the mystique of satellite internet, satellites remain far too slow and expensive for bulk transcontinental data transfer. Even with low-Earth orbit constellations like Starlink, submarine cables dominate international bandwidth — and will continue to do so for the foreseeable future.

Cable Construction: Layer by Layer

A modern submarine cable is a precisely engineered object. From the outside in, a typical cable cross-section includes:

• Polyethylene outer jacket — weathering and abrasion resistance in shallow water
• Steel wire armor — one or two layers in coastal segments; absent in deep-water sections where pressure instead of fishing trawlers is the main threat
• Copper power conductor — carries up to 15,000 volts DC to power the optical amplifiers (repeaters) spaced every 50–100 km along the route
• Aluminum water barrier — prevents seawater ingress if the outer sheath is pierced
• Polycarbonate casing — surrounds the fiber bundle
• Optical fiber strands — typically 4 to 24 fiber pairs, each pair capable of carrying a separate bidirectional stream of data

In deep water (below about 1,500 m), a cable is only about 17–25 mm in diameter — roughly the size of a garden hose. Near shore, where anchors and fishing gear pose a risk, the armor increases it to 50–70 mm.

Optical Amplification and Repeaters

Light signals weaken as they travel through fiber. To cross an ocean, cables include submerged repeaters — waterproof housings roughly the size of a large suitcase — that amplify the optical signal without converting it back to electricity. Modern repeaters use erbium-doped fiber amplifiers (EDFA), which boost signals in the 1,550 nm wavelength band.

The copper power conductor feeds a constant current (typically 0.5–1 A) down the length of the cable at high voltage. Repeaters tap off just enough power to run their amplifiers. Power is fed from shore at both ends of the cable, meeting somewhere in the middle. If one shore station loses power, the other can still supply the entire cable.

Cable Ships and Laying Operations

Laying a submarine cable is a complex, months-long maritime operation involving purpose-built cable ships. These vessels can carry thousands of kilometers of cable coiled in large tanks below decks. During laying, the cable is paid out over the stern through a cable engine that controls tension and speed as the ship moves along the planned route.

Route planning is exhaustive: hydrographic surveys chart the ocean floor, identifying mountains, fault lines, volcanic zones, and fishing activity. In shallow coastal zones, remotely operated vehicles (ROVs) and specialized plows bury the cable 1–3 meters into the seabed for protection.

A typical transoceanic cable project takes 2–4 years from initial planning to activation and costs $100–$500 million USD. A single cable system may have 5–20 investors — typically large carriers, internet companies, and national telecoms.

Landing Stations

At each end of a submarine cable is a cable landing station — a shore facility where the cable comes ashore (often via a buried conduit through the surf zone) and connects to the terrestrial fiber network. Landing stations contain:

• Shore end termination equipment — where the cable physically enters the building
• Power feed equipment (PFE) — high-voltage DC power supplies for the repeaters
• Submarine line terminal equipment (SLTE) — the transponders and wavelength-division multiplexing (WDM) gear that put traffic onto the fiber
• Operations support systems — monitoring the cable's health in real time

Landing stations are critical chokepoints. Many countries have just a handful of cable landings, meaning a single physical incident can have national-scale internet impact.

Fault Detection and Repair

Submarine cables break surprisingly often — roughly 100 faults occur globally each year. The most common causes are:

When a fault is detected (typically via a sudden change in optical power or the cable's DC loop resistance), engineers use optical time-domain reflectometry (OTDR) to pinpoint the location to within a few hundred meters. A cable ship then sails to the site, grapples the cable, lifts it to the surface, splices in a new segment, and lays the repaired cable back down. Shallow-water repairs may take 1–2 days; deep-water repairs can take 2–3 weeks.

The 300+ Cable Map

As of 2025, there are over 400 active or planned submarine cable systems worldwide. Some key examples:

• MAREA — Microsoft and Facebook's 6,600 km cable between Virginia Beach and Bilbao, Spain; 224 Tbps capacity
• 2Africa — A 45,000 km ring around Africa, funded by Meta and a carrier consortium; one of the longest cables ever built
• FASTER — A trans-Pacific cable connecting the US to Japan; 60 Tbps capacity
• SEA-ME-WE 6 — Connecting Southeast Asia, the Middle East, and Western Europe; 30,000 km
• JUPITER — A Facebook/Amazon/SoftBank cable connecting the US, Japan, and the Philippines

Interactive cable maps (such as those maintained by TeleGeography) let you explore every active system, landing station, and capacity estimate worldwide.

Why This Matters for the Internet

The physical nature of submarine cables creates real geopolitical and resilience implications:

• Chokepoints — Narrow straits like the Suez Canal, Strait of Malacca, and Luzon Strait are crossed by dozens of cables; a single incident there could affect multiple systems simultaneously.
• Repair fleet scarcity — There are fewer than 60 cable ships capable of deep-water repair globally, creating queues after major cable breaks.
• Ownership shifts — Historically dominated by telecom consortia, hyperscalers (Google, Meta, Microsoft, Amazon) now own or co-own the majority of new cable capacity, reshaping the economics of international bandwidth.
• Security concerns — Intelligence agencies and military planners monitor cable infrastructure closely; tapping and sabotage are documented concerns.

Understanding submarine cables is essential for grasping why internet latency varies between regions, why outages can be catastrophic for island nations and developing countries, and why the physical geography of the planet continues to shape the logical architecture of the internet.