Salt marshes, sand, seabirds and spies. It’s not what people normally associate with the New Forest because… it’s a forest. Isn’t it?

Well, yes. There are 193,000 acres of it, including trees that were around at the time of Henry VIII. But there’s also more than 40 miles of coastline which has been shaped every bit by the woodlands that surround it (and the industry they attracted) as by the actions of the wind and the waves upon its shores, as I discovered when I spent a day there at the invitation of the local tourism association.

“We call it the Secret Coast because in any fictional story about forests, there are mountains and rivers but never coasts so our visitors are not prepared for it and don’t seem to know it’s here,” says Anthony Climpson, the New Forest’s Employment & Tourism Manager.

The Forest has been settled for more than 6,000 years and is best-known for being officially founded as a royal hunting ground by William the Conqueror. But the coastal story is different, interweaving nature, smuggling, long-lost industry and the military as well as the popularity of its Western end as a beach destination.

All those salt-marshes, secret inlets and clear views up and down the Solent made this coast an ideal location for anything from developing sturdy fortresses, to building ships for the Napoleonic wars, to training spies and constructing concrete Mulberry Harbours during World War II.

Henry VIII first spotted its potential, building two glowering fortresses; at Calshot Spit overlooking the mouth of Southampton Water and on Hurst Spit, which stretches so far into the Solent that you feel you could almost touch the Isle of Wight.

The Forest’s secret coast was pressed into service yet again, as a shipbuilding centre, when the magnificently-named Moody Janvrin created his shipyard at Lepe in 1744. Later the focus shifted to nearby Buckler’s Hard on the Beaulieu River, where another formidable shipwright, Henry Adams, lived in what is now The Master Builder’s House Hotel.

As shipbuilding moved away, other industries moved in – who knew that for most of the 18th century, the little port of Lymington was the country’s main sea salt producer? Or that the town is the proud possessor of Britain’s first open air seawater baths – now Grade II listed and refurbished as a summer attraction?

Clues to the area’s past role as a smugglers’ haunt can be glimpsed all the way along the New Forest beaches but perhaps the most visible reminder is the brooding Watch House, a coastguard’s cottage jutting defiantly into the sea at Lepe Country Park, not far from the spot where Queen Victoria is said to have first glimpsed and fallen in love with the Isle of Wight. It was from the beaches of the Secret Coast, too, that excited onlookers would have waved and cheered the Titanic as she sailed past on her doomed voyage.

The tranquillity of this coast and the inaccessibility of much of the foreshore are what keep the wildlife coming back. This includes wildfowl, egrets, herons, birds of prey and, of course, the New Forest ponies who can occasionally be spotted rolling in sand-patches on isolated beaches. There are also the nature reserves of Keyhaven and Normandy marshes, their lonely beauty a complete contrast to the nearby holiday beaches at Milford and Mudeford.

One reason that chunks of land are virtually inaccessible is because they are privately-owned and maintained by the large estates at Beaulieu and Exbury. Today both estates are run as attractions; Beaulieu houses the National Motor Museum and Exbury is world-famous for its gardens but 70 years ago, during World War II, their very seclusion made them perfect for use by the military as a school for spies and a place to clandestinely train servicemen for the D-Day landings.

Today the coast bears proud testimony to this era; the former RAF Needs Ore Point at the tip of the Beaulieu River is a peaceful green field now, but it was an Advanced Landing Ground constructed in the summer of 1943 in preparation for the invasion of mainland Europe. Four RAF British and Commonwealth squadrons comprising some 150 aircraft were based here in the build-up to D-Day, along with over 900 ground crew. Remains of some of the D-Day Mulberry Harbours can still be seen off Lepe.

However, as Anthony Climpson points out, it doesn’t even stop with the water and the shore. ‘Divers can go beneath the waves to explore our heritage there,’ he says. Dives can be organised on the Fenna, a wooden cargo ship that sank in 1881, Serrana, a World War I cargo ship torpedoed by a U-Boat, and the SS War Knight, a supply ship torpedoed in 1918.

As with the land, the sea bed of this coast is slowly giving up its secrets. And there is something for everyone to discover.