Homeland Secure Data Network

From Govit

Also known as HSDN, this network transmits homeland security data in support of activities including intelligence, investigations, and inspections that are classified at the secret level. HSDN provides secret connectivity to civilian agencies and is to provide secret connectivity in the future for civilian agencies currently using the Department of Defense’s Secret Internet Protocol Router Network. For example, it is used to transmit intelligence summaries, secure messaging and e-mail correspondence.

(Description comes from from General Accounting Office document GAO-07-455

The GovITwiki Intelligence Agencies page lists the U.S. government's major secret and sensative but Unclassified networks.