When I stepped out of my beachfront hotel in Saint-Malo the night I arrived, I noticed a distinct lack of beach. Instead, there was only ocean: kilometres of choppy English Channel, uncomfortably close to the hotel’s entrance. But by noon the next day, the ocean was gone. As in, vanished. In its place was a beach that seemed to stretch forever. I squinted into the distance and eventually located the ocean – nearly 2km off the coast.

A walled French town on the coast of Brittany, Saint-Malo has the highest tides in Europe, with water that can rise 13m over the course of six hours. When the water goes out, it reveals several kilometres of ocean floor. But when it comes back, it comes back fast.

“You have to be careful,” the Hotel Beaufort’s receptionist, Rita Wyorek, warned me. “It comes in as fast as a person walking quickly.”

She said this with the sly confidence of someone who had perhaps raced the water just to see if she could. Large tidal variations occur up and down the English Channel, and their effects tend to be exaggerated by the shape of the bay. Saint-Malo’s location at the head of an estuary makes the phenomenon particularly dramatic because its bay fills with water from both the Atlantic and the Rance river.

While the town is hardly a secret, it’s somewhat overshadowed by its more famous sibling, Mont Saint-Michel, which attracts more than three and a half million visitors a year, compared with the little more than two million who make it to Saint-Malo. Located 55km west of Mont Saint-Michel and half an hour off the main highway, Saint-Malo tends to attract a more local crowd – a mix of French and British tourists who come for the beach and history, but end up captivated by the ocean’s wild vanishing act.

Surprisingly, the tides aren’t typically the first thing you hear about Saint-Malo. In fact, I’d come in search of a beach getaway and a peek at a medieval walled city. My hosts in France had assured me that the town was a lovely place for a dip in the ocean. They hadn’t mentioned that taking that dip is possible only during prescribed hours and after a 15- to 20-minute walk across damp sand.

The next morning, the water had dropped several metres from the night before, as though there was a slow leak in the English Channel. It was windy, and since swimming still wasn’t an option, I decided to walk along the stone ramparts into the walled part of town.

As I walked, I passed signs showing the word “DANGER”, with a person cowering beneath a giant wave. In other locations, more detailed signs warned visitors in three languages against venturing into the bay when the tide is within 10m of shore.

Near the entrance to the walled city, the ocean lapped the sidewalk while onlookers gawked. Every minute or so, a small wave would rush in, washing across the sidewalk and sending passersby scrambling to get out of the way. Half a kilometre out in the sea an old fort seemed to bob in the waves, completely encircled by water. If the world were to be swallowed by the ocean tomorrow, I thought, it would look like this.

The apocalyptic effect was only heightened by Saint-Malo’s history as a pirate stronghold. Starting in the 15th Century, the walled city’s citizens were infamous for pillaging ships crossing the English Channel. By the 18th Century, the pirates had gone legit and become corsairs – officially sanctioned pirates whom the King of France allowed to board non-French vessels in exchange for a percentage of the profits.

The town has capitalized on all this seafaring history with tourist-ready pirate hats and blue-and-white Breton-striped garments (picture a stereotypical baguette-toting Frenchman wearing a striped shirt, red neckerchief and a beret, and you’ll have an idea of Breton stripes). I found myself strolling past women wearing Breton-striped dresses, pushing strollers with babies wearing Breton-striped hats. I saw men wearing raincoats with Breton-striped lining and grown-ups dressed head-to-toe like pirates.

With its winding cobblestone streets crammed full of hotels, shops and restaurants serving buckwheat galettes (savoury crepes), cider and local Cancale oysters, Saint-Malo looks old, but it’s actually a faithful recreation of the town as it existed before August 1944. During World War II, heavy bombing and a fire flattened most of the town.

In All the Light We Cannot See, the bestselling novel set partially in Saint-Malo, author Anthony Doerr imagines what the confluence of tides and bombs must have felt like during the war: “At the highest tides, the sea creeps into basements at the very centre of town. At the lowest tides, the barnacled ribs of a thousand shipwrecks stick out above the sea. For three thousand years, this little promontory has known sieges. But never like this.”

Though the town has been rebuilt, the ocean tides, of course, remain the same.

A little after noon I headed back to the spot where the ocean had been washing over the sidewalk earlier in the day. The water was gone, sucked several kilometres offshore. The fort that had previously been inaccessible was now a short walk away. I made my way along the damp sand and inspected a few tidal pools filled with sea snails, left behind in clusters of black rocks when the ocean retreated.

The day warmed up, and as the sand dried, a small city came to life on the beach.  Tourists in wetsuits lined up for surfing lessons. Brightly coloured kites attached to surfboards, waiting to be lugged out into the ocean. Smaller, traditional kites dotted the skyline. Farther down the beach I watched wind-powered sailing cars swoop across the wet sand on a makeshift raceway. What had just a few hours earlier been all English Channel was now a full-blown beach vacation.

And then as abruptly as the tide had gone out, it began to wash back in. With the sun low in the sky, the air cooled and the surfers and land sailors left for higher ground. Wetsuit-clad lifeguards warned stragglers to leave the rapidly disappearing beach. I went to my hotel to change for dinner, and by the time I came out, the sea was back.

As I made my way along the sea wall for dinner, waves smashed against the ramparts and once again began to flood the sidewalk. Police cars blocked off a section of the road where waves were cresting several metres into the air, raining sea water down onto cars.

I took a window seat in the restaurant, oohing and aahing with the other diners every time a wave rose up and fell just short of swallowing us whole. There’s nothing like water that knows no bounds to remind you just how powerful the ocean is – and just how insignificant you and your beach vacation really are.