Roughly 60% of the Earth's outer surface is composed of oceanic crust formed by volcanic processes at mid- ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid- ocean ridges, particularly at depths below the critical point for seawater ( 3,000 m)(1). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid- ocean- ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 856 E, to acquire photographic and video images of 'zero- age' volcanic terrain on this remote, ice- covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele)(2), covering a large ( > 10 km(2)) area. At least 13.5 wt% CO(2) is necessary to fragment magma at these depths(3), which is about tenfold the highest values previously measured in a mid- ocean- ridge basalt(4). These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge(5) and demonstrate that large- scale pyroclastic activity is possible along even the deepest portions of the global mid- ocean ridge volcanic system.