Hollow-ridge microrelief is a typical surface pattern of raised bogs in the Boreal zone. It may play an essential role in maintaining the balance between bog ecosystems and changing environmental conditions. The data of the location, depth, approximate age, area and length/width ratio of 225 hollows and reconstructions of the shape dynamics of eight hollows from six bogs in Estonia were used to study the dynamics of the formation and development of hollows. The approximate age of hollows was established according to their depth and peat age isolines calculated by the bulk density method and based on radiocarbon dating. The first period of intensive hollow formation in the bogs studied occurred 3200–2800 years ago. The second, still continuing period of intensive hollow formation started c.1800 years ago. No correlation was detected between the temporal distribution of hollows on different profiles within one bog or in different bogs. Therefore, the formation of the hollows could have been initiated by extra-local random factors supported by the cool wet climate. Almost synchronous widening of the area of hollows in different bogs in Estonia about 1600–1400, 1100–900 and 600–400 years ago is contemporaneous with cool and or wet climatic periods and was probably initiated by climatic changes.
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