Bog boies, also known as bog people, are preserved human bodies found in sphagnum bogs in Northern Europe, Great Britain and Ireland. Unlike most ancient human remains, bog bodies have retained skin and internal organs due to the unusual conditions of preservation. Under certain conditions, the acidity of the water, the cold temperature and the lack of oxygen combine to tan the body's skin: skeletal preservation is very rare in these bodies, as the acid in the peat dissolves the calcium carbonate of bone. The bodies provide very useful research material for archaeologists. Some of the bodies retain intricate details like tattoos and fingerprints. C.H. Vogelius Andersen was astonished to find that Grauballe Man's hand prints were clearer than his own. The stubble and facial features of Tollund man are particularly well preserved.