The scope of the National Security Agency's surveillance capabilities is as good as anybody's guess. But the latest batch of revelations suggests the shadowy organization has a cheeky streak.

The Washington Post reports that that NSA has secretly broken into "the main communications links" of the web's top two services — Yahoo and Google — and their data centers around the world, according to documents swiped by NSA leaker Edward Snowden.

This allows the NSA to gather vast amounts of user information from services like Gmail, Google Docs, and Google Maps, copying "entire data flows across fiber-optic cables that carry information between the data centers of the Silicon Valley giants," says the Post. In addition to meta-data, the NSA can glean who sent or received emails, as well as content such as "text, audio, and video."

The document also includes this smiley-faced Post-it note, apparently scribbled by one NSA employee for another:

The Post reports that "two engineers with close ties to Google exploded in profanity when they saw the drawing."

The document, dated Jan. 9, 2013, shows that the NSA's primary tool for hacking into the data links connecting Google and Yahoo and their data centers is a project called MUSCULAR, which is shared with the agency's British counterpart GCHQ. The program is separate from PRISM, which provides the NSA with front-door access through legal means.

MUSCULAR is different. Its primary channel is via a shady back-end, yet similarly allows the NSA to gobble up enormous amounts of data.

By tapping those links, the agency has positioned itself to collect at will from among hundreds of millions of user accounts, many of them belonging to Americans. The NSA does not keep everything it collects, but it keeps a lot. [Washington Post]

A spokeswoman for Yahoo says: "We have strict controls in place to protect the security of our data centers, and we have not given access to our data centers to the NSA or to any other government agency." Similarly, a Google representative says that the company is "troubled by allegations of the government intercepting traffic between our data centers, and we are not aware of this activity."

Gen. Keith Alexander, the NSA's head, denied the allegations in the latest report, saying the NSA had not authorized such data collection.