France has launched a huge project to remove silt linking the mainland to Mont Saint-Michel - a national landmark - and make it an island again.
Back in 1884 writer Victor Hugo said it "must remain an island"
The build-up of silt has spoiled the insular character of the Benedictine abbey perched on a rock off the north-western Normandy coast.
Engineers will replace an old dyke with a bridge and build a hi-tech dam to get the water flowing again.
The work should be completed in about 2012, the French news agency AFP says.
The unusual abbey and steepled church of Mont Saint-Michel were built between the 11th and 16th centuries. It is a Unesco world heritage site.
"Historically, the Mont was more than four kilometres (2.5 miles) from the mainland. Today it is only a few dozen metres (yards) away," said project director Francois-Xavier de Beaulaincourt.
The project will cost 150 million euros (£103m; $190m). It was launched by Prime Minister Dominique de Villepin on Friday.
The hydraulic dam is to allow tidal waters to flow freely up the nearby Couesnon river, but should filter out much of the sediment. When the tide goes out the dam should then release water to flush silt away from the river delta.