The British surveillance programs have fittingly sinister titles: "Mastering the Internet" and "Global Telecoms Exploitation," according to The Guardian.

A subsidiary program for these operations -- Tempora -- sucks up around 21 million gigabytes per day and stores the data for a month. The data is shared with NSA, and there are reportedly 550 NSA and GCHQ analysts poring over the information they've gathered from at least 200 fiber optic cables so far.

The scale of the resulting data harvest is tremendous. From The Guardian:

This includes recordings of phone calls, the content of email messages, entries on Facebook and the history of any internet user's access to websites -- all of which is deemed legal, even though the warrant system was supposed to limit interception to a specified range of targets.

In an interview with online security analyst Jacob Appelbaum, NSA leaker Edward Snowden called the British spy agency GCHQ "worse than" the NSA, saying it represents the first "full take" system, in which surveillance networks catch all Internet traffic regardless of its content. Appelbaum asked Snowden if "anyone could escape" Tempora:

"Well, if you had the choice, you should never send information over British lines or British servers," Snowden said. "Even the Queen's selfies with her lifeguards would be recorded, if they existed."

The U.S.'s own cable-tapping program, known by the names OAKSTAR, STORMBREW, BLARNEY and FAIRVIEW, as revealed in an NSA PowerPoint slide, apparently functions similarly to Tempora, accessing "communications on fiber cables and infrastructure as data flows past," according to The Washington Post. The slide indicates that Prism and these so-called "upstream" programs work together somehow, with an arrow saying "You Should Use Both" pointing to the two operations.

So how does one tap into an underwater cable?

The process is extremely secretive, but it seems similar to tapping an old-fashioned, pre-digital telephone line -- the eavesdropper gathers up all the data that flows past, then deciphers it later.

More than 550,000 miles of flexible undersea cables about the size of garden watering hoses carry all the world's emails, searches, and tweets. Together, they shoot the equivalent of several hundred Libraries of Congress worth of information back and forth every day.

In 2005, the Associated Press reported that a submarine called the USS Jimmy Carter had been repurposed to carry crews of technicians to the bottom of the sea so they could tap fiber optic lines. The easiest place to get into the cables is at the regeneration points -- spots where their signals are amplified and pushed forward on their long, circuitous journeys. "At these spots, the fiber optics can be more easily tapped, because they are no longer bundled together, rather laid out individually," Deutsche Welle reported.