Deep beneath the ice-sheathed Arctic Ocean, a 1,000-mile seam in the earth's rocky crust, long thought to be largely dormant, has been revealed as a simmering necklace of volcanoes and hot-water vents that may harbor unique life forms. Earlier surveys in the depths near the North Pole had identified a couple of seabed volcanoes in one place along the seam, which is called the Gakkel Ridge and bisects the polar ocean from Greenland to Siberia.But sonar and seismic readings, and rock and water samples gathered during a recent jojnt expedition by German and American ice-breaking ships, have created a detailed overview of the surprisingly dynamic geology of the ridge.