The wealth and influence of the abbey extended to many daughter foundations, including St. Michael's Mount in Cornwall. However, its popularity and prestige as a centre of pilgrimage waned with the Reformation, and by the time of the French Revolution there were scarcely any monks in residence. The abbey was closed and converted into a prison, initially to hold clerical opponents of the republican regime. High-profile political prisoners followed, but by 1836, influential figures—including Victor Hugo—had launched a campaign to restore what was seen as a national architectural treasure. The prison was finally closed in 1863, and the mount was declared an historic monument in 1874. Mont Saint-Michel and its bay were added to the UNESCO list of World Heritage Sites in 1979, and it was listed with criteria such as cultural, historical, and architectural significance, as well as human-created and natural beauty.[3]