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The Portal ARG, or Alternate Reality Game, was introduced as an easter egg in the March 1 Patch. The notes released along with the patch read "Changed radio transmission frequency to comply with federal and state spectrum management regulations", and accompanying this was a new achievement, "Transmission Received".
In order to acquire the achievement, players were required to collect a Radio in each test chamber, and carry it to a specific location. The red light on the radio would change to green when the correct location was reached, after which the default music would change to a series of beeps, similar to Morse Code. If a player carried the radio through a Material Emancipation Grill, a different sound would play. It was later discovered these sounds, stored in 26 separate WAV files, contained hidden images that could only be accessed by using Slow-scan television software.
In addition to images, the WAV files also contained brief messages in Morse Code, as well as a phone number and a username/password combination. It was eventually discovered that dialling the number using an analogue modem lead to a GLaDOS-themed BBS which, when accessed using the aforementioned username and password, revealed more images and other data that hinted that Portal 2 was forthcoming.
On March 3, another patch was released that changed the ending of Portal, and this was considered confirmation that Portal 2 would soon be released. On March 5, three days after the commencement of the ARG, the sequel was officially announced.