The Hawai'i Undersea Geo-Observatory (HUGO) is an ocean bottom observatory located on the summit of Lo'ihi seamount, Hawai'i. An electro-optical cable connects the HUGO junction box to a shore station on the Big Island of Hawaii, thereby enabling the first real-time monitoring of a submarine volcano. HUGO was active for 3 months in 1998, collecting nearly continuous, real-time data on a high-rate hydrophone. Signals detected during that time include local as well as teleseismic earthquakes, T phases from Pacific-wide earthquakes, landslides on the submarine flank of Kilauea, and eruption sounds from the current Kilauea eruption. The data do not indicate a Lo'ihi eruption during the time that HUGO was active. The variety and quality of signals detected by the HUGO hydrophone confirms that a real-time observatory can serve a valuable role in studies of oceanic acoustics, local and teleseismic earthquakes, and submarine eruption mechanics.