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The
University of Washington's Hubble system tracks unreachable areas of the internet in real time.
From their website:
Having trouble accessing a favorite Web site? Perhaps the site was taken offline, or the computer hosting it is down for maintenance. However, the cause could be something more mysterious. At any given moment, a portion of Internet traffic ends up being routed into information "black holes." These are situations where advertised paths exist to the destination, but messages - a request to visit a Web site, an outgoing e-mail - get lost along the way. Hubble is a system that operates continuously to find persistent Internet black holes as they occur.