This is the Third Street Tunnel in Washington DC, better known as the Tunnel of Doom. This is the tunnel where thousands of purple ticket holders were forced to line up to witness the First inauguration of Barack Obama.
There were so many people wanting to celebrate and experience the event that the tunnel soon became overcrowded with thousands of people. Reports suggest the actual number of people was between 1 and 4 thousand.
Many of these people actually never got to see the event but despite this, they all managed to group together and start to sing. Once one person started, the rest of them joined in, until very soon thousands of people were singing.