Why is the standard depth of a grave six feet?

Mound 1 at Sutton Hoo is dated to circa 625, based on a single dated gold coin found with the treasure to Frankish king Theodebert II (595-612). The ship was about 27 meters (89 feet) long and 4.2 meters (14 feet) wide amidships and would have been powered by 40 oarsmen. It was placed in the mound with the bow facing to the East and away from the river. A burial chamber was constructed mid-deck and, based on the arrangement of the armour, the body within was laid with its feet to the East and head to the West. There were no remains of a body found at the time of the initial excavation but more recent analysis has found chemical traces showing that a body had been present. The most likely candidate for the burial is King Rædwald.