Undersea volcano: Heard but not seen.

Seismologists in Oregon have been eavesdropping on the rumblings of a mysterious submarine volcano since May 1998, but they haven't been able to locate the loudmouth.

The scientists, from the National Oceanic and Atmospheric Administration (NOAA), use a network of underwater microphones, or hydrophones, to listen to the ocean. Mainly for detecting earthquakes, the hydrophones can pick up whale calls and construction projects an ocean away, says seismologist Robert P. Dziak of NOAA and Oregon State University in Newport.

The unseen volcano has a distinctive voice, he reports. Its soundprint is unusually regular. This initially led Dziak to think the sound might be human- or even whale-made. However, it was so loud and long-lasting that he concluded that the rumble resembles those from other active land and sea volcanoes.

Dziak used data from hydrophones scattered around the Pacific to try to find the eruption's location. His calculations indicate that it lies about 1,000 kilometers south of Honshu Island, Japan, along the volcanically active Bonin trench.

Unfortunately, the hydrophone network doesn't extend west of where the volcano appears to lie, making it impossible to triangulate the position. "We don't have the smoking gun, so to speak," says Dziak, but he hopes other researchers have data that could point him in the right direction.

Two candidates for the mystery volcano--the submerged volcanoes Fukutoku-okanoba and Funka-asane--stand at the edge of the search area. Discolored seawater seen over the volcanoes could indicate eruptions, but the timing of water-color changes doesn't match the rumbling of the unknown volcano.

Dziak reported his search for the growling volcano in the most recent issues (November and December 1999) of the BULLETIN OF THE GLOBAL VOLCANISM NETWORK.