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The fibre on the ocean floor that carries almost all the world's data between continents. A huge fixed build, cable, marine lay and landing stations, then a capacity business: sell the lit terabits (Tbps × fill × price per Gbps), increasingly on long-term and IRU contracts so a large share of revenue is contracted, for decades. Pick a real system below and follow it through the build, the model and a working returns model.
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Drag the sliders to see what earns the money, the lit capacity, the price per gigabit, and how fully the cable is sold (fill), not the raw bandwidth.
A submarine cable is a capacity business on top of a huge fixed build. The capital, the cable on the seabed, the marine lay, the landing stations and the lit terminal equipment, is enormous and almost entirely sunk. Against that, revenue is lit capacity × fill × price: how many terabits per second are lit, how much of that is sold, and the price per gigabit per month. The catch is that unit prices fall every year as technology improves, even as bandwidth demand grows, so the operator lives on filling the cable and locking in long-term IRU (indefeasible right of use) contracts. The big shift in the market is who buys and builds: the hyperscalers (Google, Meta, Amazon, Microsoft) now fund and own a growing share of cables for self-supply, and increasingly contract capacity long-term, so more and more subsea revenue is contracted (a high floor) rather than merchant. Operating cost is low and largely fixed, so once a cable fills the margin is very high.
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See also the Subsea cable simulator, lay and operate a cable interactively, and the Cash-flow & DCF model.