Dry fogs appear in the atmosphere when large volcanic eruptions inject massive quantities of fine silicate ash and aerosol-forming sulphur gases into the troposphere and stratosphere. Although the ash gravitationally settles out within weeks, the aerosols spread around the globe and can remain suspended in the stratosphere for years. Because solar radiation is easily absorbed and backscattered by the volcanic particles, a haziness in the sky and a dimming of the Sun and Moon are produced. Very dense and widespread dry fogs occur, on the average, once every few centuries1-3. The sizes and intensities of some of the largest of them before the modern scientific era have been estimated by several indirect methods2,4,5. The densest and most persistent dry fog on record was observed in Europe and the Middle East during AD 536 and 537. Despite the earliness of the date, there is sufficient detailed information to estimate the optical depth and mass of this remarkable stratospheric dust cloud. The importance of this cloud resides in the fact that its mass and its climatic consequences appear to exceed those of any other volcanic cloud observed during the past three millenia. Although the volcano responsible remains a mystery, a tropical location (perhaps the volcano Rabaul on the island of New Britain, Papua New Guinea) can be tentatively inferred.