Certain researchers investigating the Dogon have reported that they seem to possess advanced astronomical knowledge, the nature and source of which have subsequently become embroiled in controversy. From 1931 to 1956, the French anthropologist Marcel Griaule studied the Dogon. This included field missions ranging from several days to two months in 1931, 1935, 1937 and 1938[29] and then annually from 1946 until 1956.[30]