Lake Tana has a number of islands, whose number varies depending on the level of the lake. It has fallen about in the last 400 years. According to Manoel de Almeida (a Portuguese missionary in the early 17th century), there were 21 islands, seven to eight of which had monasteries on them "formerly large, but now much diminished." When James Bruce visited the area in the later 18th century, he noted that the locals counted 45 inhabited islands, but stated he believed that "the number may be about eleven." A 20th-century geographer named 37 islands, of which he believed 19 have or had monasteries or churches on them.