Recent excavations proved that those were full figure statues, buried up to the neck. Why Easter Islanders bothered to do that is unclear to me. Sunken?
They didn’t — the statues were on platforms, the vegetation died off, rainy season led to erosion. Also the culture of the island changed; a rebellion resulted in a lot of statues tipped, or the topknots knocked off at the very least.
http://www.freerepublic.com/focus/news/2883427/posts?page=70#70
http://www.freerepublic.com/focus/news/1714661/posts