Walk Of The Month: Water's out, moon's up - go for a night hike on the seabed

At low tide, Jersey doubles in size. Carolyn Fry discovers the underwater secrets exposed by the lunar cycle

'Welcome, fellow lunatics," grins Andrew Syvret, as a group of us gather for a guided walk on Jersey's foreshore. Nothing crazy about that you might think - except that it is pitch black and one o'clock in the morning. We are here at this time because it is low tide, which means we can explore the undulating landscape of the channel island's rocky seabed. With the fourth highest tidal range in the world, Jersey doubles in size to 116 square miles every 12 hours when the English Channel drains away. The landscape revealed by this fierce tidal flow looks distinctly lunar, according to Andrew, who is a marine biologist. So what better way to explore it than by the light of a full moon?

As we step off the stone slipway, the moon is struggling to shine through patches of cloud. Apart from the circles of light cast by our head torches, the only real illumination comes from orange pinpricks of streetlights further up the coast and the sweeping beam of a distant lighthouse. Slowly, as our eyes adjust, we can make out shallow pools and dark shadows of rocks covered in glistening seaweed.

After 15 minutes or so, the firm sand underfoot changes to gravel, a sign that we have moved beyond the shelter of the island into a higher-energy environment. A few more steps and we find ourselves crunching through a limpet graveyard. The shells of mature limpets have been sorted and dumped by the currents that circle the island. "The water moves anti-clockwise, sculpting and sifting material along the way," Andrew explains. "There are lots of distinct messages in the seascape that tell us which way the water has been blown. It's never the same, not even on two consecutive tides."

The speed and strength of the tidal flow make Jersey's shore a dangerous place for the uninitiated. When the the tide comes in it does so in a pincer movement, racing unseen along sunken gullies to cut off the unsuspecting. After 30 minutes or so, we reach a 40ft high metal platform on spindly seaweed-draped legs, which Andrew describes as the "point of no return". Anyone finding themselves trapped by the tide has to climb to the top of the tower as quickly as possible and await rescue. "The tide comes in at five vertical centimetres [about two inches] a minute," he warns. "The difference between getting traction on the gravel or being swept away can be as little as 20 or 30 seconds. There is no mercy out here. It's a savage, brutal and unforgiving environment."

For a thousand years Jersey islanders have risked their lives on the seabed to earn a living from harvesting seaweed for fuel and fertiliser. In the industry's heyday, workers would bring horses and carts to the shore and pile them high with as much of the slippery brown stuff as they could cut. Though the industry waned after the introduction of agrochemicals in the 1950s and 1960s, evidence of it remains in the seascape. We are more than a mile out when we come across a stone tablet, engraved with the letter "P". "This is the initial of the Payn family," Andrew explains. "There are six of them that mark a boundary along the seabed. They were placed by order of the Royal Court of Jersey in 1747 to confirm the Payn family's exclusive rights to harvest seaweed here."

Today, the Channel Islands' most lucrative harvests are of fish and shellfish. At high tide the gullies along which we are walking are crowded with sea bass. The shallow sea, which heats up to 23C (76F) in the summer, acts as the English Channel's incubator, nurturing the lobster, crab, and oysters that we love.

The looming silhouette of the square Seymour Tower - built in 1782 to defend Jersey - marks the farthest point of our two-and-a-half-hour walk. We climb its granite steps and Andrew hands round warming nips of Calvados, and Jersey buns. A ribbon of moonlight reveals the impatient sea in the not-so-far distance. We need to get back. Taking a last peek towards France, we clamber down and retrace our steps. In no time at all the sea will be galloping back to wash away our footprints and return the seabed to the creatures that make it their home.

GIVE ME THE FACTS

Distance: 3 miles

Time: 2.5 hours

Andrew Syvret (01534 485201) runs regular day and night-time "moonwalks" across Jersey's seabed. Attempting such walks without a guide is dangerous because of the fast tidal flow. The moonwalks are one of a wide selection of guided walks taking place during Jersey's Autumn Walking Week, 18-25 September. For dates and times, plus bus timetables, contact Jersey Tourism (01534 500777; www.jersey.com). Carolyn Fry flew to Jersey with VLM. Flights available from London City Airport (020-7476 6677; www.flyvlm.com), with connections from Manchester and Liverpool. Returns from £56.20 in August.