One of the ways that the US National Security Agency taps into internet traffic is by going straight to the major data hubs that international traffic passes through. But secretly gathering that data isn't so simple: for every tap into the transmissions, some data is lost — and someone monitoring it for intruders is bound to notice.
According to The Atlantic, the NSA gets around that by using a special tool designed to avoid data loss. Because the data is traveling as light along fiber optic cables, the NSA employs a prism that can split the light into two separate beams. The prism allows the primary beam of data to continue along with only the slightest loss of strength, while a faint — but still readable — second beam is captured by the NSA for decoding. The Atlantic has more on the NSA's methodology, along with a short history of how the US government began tapping into cables in the first place.