A bog body is a human cadaver that has been naturally mummified in a peat bog. Such bodies, sometimes known as bog people, are both geographically and chronologically widespread, having been dated to between and the Second World War. The unifying factor of the bog bodies is that they have been found in peat and are partially preserved; however, the actual levels of preservation vary widely from perfectly preserved to mere skeletons.
Unlike most ancient human remains, bog bodies often retain their skin and internal organs due to the unusual conditions of the surrounding area. Combined, highly acidic water, low temperature, and a lack of oxygen preserve but severely tan their skin. While the skin is well-preserved, the bones are generally not, due to the dissolution of the calcium phosphate of bone by the peat's acidity. The acidic conditions of these bogs allow for the preservation of materials such as skin, hair, nails, wool and leather which all contain the protein keratin.
The oldest known bog body is the skeleton of Koelbjerg Man from Denmark, who has been dated to 8000 BCE, during the Mesolithic period. The oldest fleshed bog body is that of Cashel Man, who dates to 2000 BCE during the Bronze Age. The overwhelming majority of bog bodies – including examples such as Tollund Man, Grauballe Man and Lindow Man – date to the Iron Age and have been found in northwest Europe, particularly Denmark, Germany, the Netherlands, United Kingdom, Sweden, Poland, and Ireland. Such Iron Age bog bodies typically show a number of similarities, such as violent deaths and a lack of clothing, which has led archaeologists to believe that they were killed and deposited in the bogs as a part of a widespread cultural tradition of human sacrifice or executed as criminals. Bogs could have indeed been seen as liminal places positively connected to another world, which might welcome contaminating items otherwise dangerous to the living.