Almost all the countries in the world expect their foreign intelligence services to tap and sift through international telecommunications. For that reason, network operators whose lines cross international borders are legally obliged to make certain intersection points available to the authorities. Britain's Tempora program, for instance, had perfectly legal access to the information it obtained - at least when it passed through British territory.

From electricity to light, and back

But fiber-optic cables can also be tapped secretly, without the knowledge of the operators - though this is not exactly easy. To understand how it works, one has to look more closely at how the data actually passes through the cables.

A standard fiber-optic cable laid across land consists of 144 individual glass fibers, while undersea cables consist of a maximum of eight individual fibers. Using laser technology, the electronic data is initially turned into ultra-short flashes of light. These flashes represent the zeros and ones that all digital information is comprised of. A photodiode at the end of the cable turns the light flashes back into electrical signals.

Around 10 billion such flashes of light run through these cables every second, and each one can also transfer between 1.2 and 5 gigabytes of data per second. But since the capacity of fiber optics is never completely used up, in practice the data flow is usually equivalent to between one and five standard CDs.

Fiber optics need amplifiers

But after a certain distance, the data signal drops. Every 80 kilometers or so, the signals have to be re-amplified, explained Klaus-Dieter Langer of the Fraunhofer Heinrich-Hertz-Institute in Berlin.

This is done with the help of a "regenerator." Undersea cables also have regenerators, which are supplied with electricity by copper cables laid across the ocean floor, together with the fiber optics.

These regenerators are the system's weak point. At these spots, the fiber optics can be more easily tapped, because they are no longer bundled together, rather laid out individually (since each fiber must be amplified separately). At these points, data piracy is not necessarily easy - but that, as Langer puts it, is "just a technical hurdle."

A vigilant network operator can spot such hacking attempts. "You need very sensitive measuring instruments," said Langer, "then you can see when the signal strength suddenly dips."

Order in the data chaos

Once a spy has succeeded in hacking into a cable, the bigger challenge emerges - sifting through the immense mass of data. This needs to be done quickly. Even if a single glass cable is operating only at 50 percent capacity, it can still deliver 10 terabytes of data in an hour. "Since storage capacity is finite, the trick is to analyze these 10 terabytes within an hour, and filter out what you're looking for," said Langer.

A lot of the data needs to be decrypted - which also means being temporarily stored. At the same time, intelligence agencies must proceed very selectively so as not to get bogged down in the flood of data. Langer believes that agents probably concentrate on single fibers belonging to certain operators of particular interest. "It makes more sense to search for certain content, rather than, for example, email conversations, telephone connections and the like.

Wire-tapping contest under the ocean

Hacking a cable only makes sense if you have large server capacity immediately available, which is why Langer is skeptical of recent media speculation about the USS Jimmy Carter, a nuclear submarine said to be on a mission to tap underwater cables. "It seems bizarre," said Langer.

But Peter Franck, spokesman for the Chaos Computer Club digital rights collective, considers the submarine reports "absolutely believable." Though tapping underwater cables is so secret "that it would never be publicly talked about," so far reports in the American media have not been denied by the government.

Franck can imagine a number of ways in which data could be moved from the submarine to servers on shore. He speculates, for instance, that the data could be pre-filtered on board and then broadcast to a base via the normal radio communication. Or a device that records the data could be left on the ocean floor. "An extra vehicle could then come and pick it up," Franck suggested.

Such underwater cables are certainly of considerable interest to intelligence agencies, since a huge part of international communication travels through them. It could certainly be the case that a lot of the world's fiber optic cables are being tapped - and not only in countries where respective intelligence agencies are based.