Long-term monitoring has revealed variations in the temperature and salinity of bottom water overlying the axis of the Mid-Atlantic Ridge. These are attributed to three primary mechanisms: externally introduced changes in the water mass: topography-driven vertical mixing: and geothermal heating. Here, we focus on anomalous temperature changes that were induced within the ridge axis. Over a period of 35 days the bottom water warmed by up to 0.05 ¿C but without changing its salinity. During the following 60 days, anomalously warm water was advected out of the southwestern end of the ridge axis at a depth of 2850 m. We attribute the temperature anomaly to a geothermal event, triggered by either magmatic or tectonic activity, during which ~6.9¿1015 J of heat were dissipated. This is equivalent to the emplacement and cooling of 1.5¿0.15¿106 m of basaltic material on, or into, the shallow oceanic crust. Alternatively, it could have been caused by the release of an upper crustal reservoir of warm. 50 ¿C above ambient, with a volume of 3.4¿107 m3. ¿ 2000 American Geophysical Union