Monitoring any portion of the Internet address space reveals incessant activity. This holds even when monitoring traffic sent to unused addresses, which we term "background radiation." Background radiation reflects fundamentally nonproductive traffic, either malicious (flooding backscatter, scans for vulnerabilities, worms) or benign (misconfigurations). While the general presence of background radiation is well known to the network operator community, its nature has yet to be broadly characterized. We develop such a characterization based on data collected from four unused networks in the Internet. Two key elements of our methodology are (i) the use of filtering to reduce load on the measurement system, and (ii) the use of active responders to elicit further activity from scanners in order to differentiate different types of background radiation. We break down the components of background radiation by protocol, application, and often specific exploit; analyze temporal patterns and correlated activity; and assess variations across different networks and over time. While we find a menagerie of activity, probes from worms and autorooters heavily dominate. We conclude with considerations of how to incorporate our characterizations into monitoring and detection activities.