As part of its domestic spying program, the NSA may have gained access to Google and Yahoo's servers by "tapping" the fiber optic cables that connect the world's major data centers, reports the New York Times.

These cables are owned by private companies that help facilitate the daily grind of making the Internet work. One such company is called Level 3 and counts both Google and Yahoo as customers.

An anonymous source for the NYT said, "The Internet companies’ data centers are locked down with full-time security and state-of-the-art surveillance, including heat sensors and iris scanners. But between the data centers — on Level 3’s fiber-optic cables that connected those massive computer farms — information was unencrypted and an easier target for government intercept efforts."

When they realized that this may be the means the NSA was accessing their data, Google and Yahoo began encrypting the data then transmit over these cables. They point to their doing so as proof that they aren't cooperating with the NSA to spy on citizens.