This article explains how the National Security Agency (NSA) was probably able to monitor internet traffic by tapping into the internet backbone. It also describes the history of the tenacity with which the NSA had pursued its aim to monitor communications.
The internet backbone physically means “the fiber-optic cables that connect data centers around the world and are owned by companies like Verizon Communications, the BT Group, the Vodafone Group and Level 3 Communications.”
Although data centres of the big internet companies are secured, “Level 3’s fiber-optic cables that connected those massive computer farms — information was unencrypted and an easier target for government intercept efforts […].”
“Although the Internet is designed to be a highly decentralized system, in practice a small group of backbone providers carry almost all of the network’s data. ”