The mystery of Wiuwert (Wiewerd)

Before going home we decided on a last visit. We went to a little old church that we've been to with our parents as well, about 50 years ago. In a cellar in that church are the remains of four persons. All of them mummified. The persons died around 1600 and were discovered in 1765 when someone out of curiosity opened a casket and saw a mummified person in stead of bones. The bodies were mummified by natural circumstances, unique in the world. Nobody knows why that happened. Humidity, temperature or the constant flow of fresh air? 
It's a mystery...