If you want to drive from Beauvoir-sur-Mer to Barbâtre on the western coast of France, you have a couple of options. You can go southwest on Route D22 to La Barre-de-Monts and catch Route D38, taking it north to your destination for a total distance of 18 km (11 mi). The alternative is to take the Passage du Gois directly to Barbâtre and shave 6 km (3.73 mi) off your journey. Why on earth would you consider taking the longer route? It might have something to do with the fact that the latter road is submerged twice a day under the waters of the Atlantic Ocean.

Passage du Gois at low tide (left) and high tide (right).

Passage du Gois was built in the 16th century to connect the Gulf of Burnёf with the island of Noirmoutier. The 4.15 km (2.58 mi) causeway is only accessible from 90 minutes before the lowest tide of the day until 90 minutes after. When high tide comes in, Passage du Gois can be as much as 4 meters (13 feet) below the surface.

For those who are unfortunate enough to get caught in the middle when the tide comes in, the road designers were considerate and provided a few towers, allowing a safe haven until rescue comes or the tide recedes. As for your car, well, you’d better hope you left the windows rolled up.

When the road reappears, it is frequently covered in seaweed, making it quite slippery at times. It was for this reason that Passage du Gois was temporarily taken off of the Tour De France route after a 1999 accident during the world-famous bicycle race. Just as it disappears and reappears regularly, it has subsequently reappeared in the event, again being included in the 2011 race.

Authorities attempt to recover a vehicle that was swept off the Shell Island Causeway during high tide.

Passage du Gois is not the only road that tries to imitate the lost continent of Atlantis. Shell Island, also known as Mochras, lies west of Llanbedr in Wales. Although called an island, it is a peninsula. It obtained “island” status when the Earl of Winchelsey diverted the River Artro in 1819 from its previous course where it entered the sea to the south of Shell Island. Vehicular access to Shell Island is by a causeway that, like Passage du Gois, disappears under the tide twice each day. Like other roads of this nature, travelers frequently give in to the temptation to race the tides, only to find themselves stranded and deprived of a vehicle.

Lindisfarne Castle on the island of Lindisfarne at high tide (top) and low tide (bottom).

Approximately 400 km (250 mi) northeast of Shell Island you will find another disappearing roadway. Holy Island, also known as Lindisfarne, is in Northumberland County just a few miles south of the border with Scotland. Lindisfarne is known for being home to Lindisfarne Castle, a 16th-century structure that sits as the highest point of the island. Home to a community of 180 people, Lindisfarne Island is 1.6 km (1 mi) from the mainland and is accessible by foot and vehicle via a causeway, from about three hours after high tide until two hours before the next high tide. Despite numerous posted warnings about the danger of venturing into the area affected by the tides, the Coastguard ends up rescuing a stranded vehicle about once each month.

Låningsvejen Causeway to Mandø (left) and the Mandø Bus (right).

Nearly 725 km (450 mi) due east of Lindisfarne is the Danish island of Mandø. Like the preceding examples, Mandø is cut off from the mainland by high tide and accessible by a causeway at low tide. What is different about this causeway, known as Låningsvejen, is that it is unpaved. It is a gravel roadway that crosses the massive mudflat. Although technically open to all motor vehicles, the specific challenges of this route cause many visitors to choose to travel via a specially-designed tractor known as the “Mandø Bus.”

If you find yourself motivated by the Robert Frost poem to consider the road not taken, one of these routes may appeal to you. Just remember the old adage that time and tide wait for no one. If your timing is wrong when you travel one of these routes, you’ll find yourself up a creek without a paddle.

Read more fun facts about geography.

Read more fun facts about transportation.