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No feature on the entire Welsh coastline is more remarkable or more fascinating than the sunken valley of .

Shakespeare, while he may not have visited the area, certainly knew of it. In Cymbeline he wrote about the ria (to give the waterway its correct geological name):

"Tell me how Wales was made so happy
As to inherit such a Haven."

The area around Milford Haven has felt and seen the presence of human beings since man first trod upon the earth. Evidence of early people has been found in many of the caves that nestle into the carboniferous limestone outcrops around the Haven, and it does not take the greatest imagination in the world to conjure a vision of hunting people from the Mesolithic and Neolithic periods paddling or sailing up the estuary in their simple skin boats, boats that were not dissimilar to modern Welsh .

Welsh coracle

Later, the Romans also knew the Haven and probably used it as a temporary base for their patrolling fleets from what is now the Cardiff area.

When the Romans left Britain in the years after 410 AD the early Christian missionaries used the estuary as a route into mainland Britain but it was the raiding Vikings that left an really indelible mark on the waterway.

Between AD 844 and 1091 they raided the Pembrokeshire coast many times, burning the nearby cathedral at St Davids on no fewer than eight separate occasions. Milford Haven was a safe anchorage for the Norsemen and the Chieftain Huba almost certainly gave his name to Hubberston, a small village at the mouth of the estuary, after he spent the winter of AD 877 sheltering in the Haven. Huba was apparently accompanied by a fleet of over 20 ships and nearly 2,000 warriors. The effect on the local birth rate can only be imagined.

Cleddau River in the fog. Photograph by George Johns.

Several other locations in and around the Haven have names of Viking origin. Carr Rocks off the town of Pembroke Dock derive their name from the Norse word "scare," meaning rocks, while Skokholm and Skomer Islands, just outside the Haven, are clearly names of Norse derivation.

With the coming of the Normans - themselves a people of Norse origin - the area of Milford Haven began to assume even greater political and military significance. A series of strong stone castles across the centre of Pembrokeshire created a line or barrier, the Landsker, with the Welsh to the north and a mixture of Flemish, Welsh and English to the south. Milford Haven lay within the English speaking region to the south of this Landsker and was, therefore, hugely significant for the early Norman kings in their campaigns against the Welsh and Irish.

The Haven has often been a centre for invasion, both outwards and inwards. In 1171 it was the base for Henry II's invasion of Ireland, over 400 ships gathering in the estuary before the assault.

In 1397 Richard II also left for Ireland from the Haven, as did Oliver Cromwell in 1649. Coming the other way, the Haven saw the arrival of a large number of French mercenaries, journeying to support Owain Glyndŵr, in his rebellion in 1405. Henry Tudor, born in Pembroke Castle just off the waterway, also landed in the Haven when he came to challenge and defeat Richard III in 1485.

The Haven had been used as a significant port since the Middle Ages but the modern-day town of Milford did not exist until 1790 when Sir William Hamilton, husband of Nelson's Emma, founded the place. His nephew, Charles Greville, invited seven Quaker whaling families from Nantucket and New England to settle in the town and start a whaling fleet.

The whaling venture was short lived but Milford did become an important fishing centre. By 1906 it was the sixth largest fishing port in Britain with over 500 people working either in the industry itself or in related trades. The fishing fleet continued to thrive throughout the first half of the 20th century, only really beginning to decline once fish stocks in the Atlantic started to vanish in the 1950s.

The importance of the Haven as a port had been noted by Admiral Nelson during his visit to the area in 1802, the Admiral apparently declaring it one of the finest natural harbours in the world. Considering his relationship with Emma Hamilton - and Sir William, with whom they lived in a bizarre ménage a trois - if Nelson did make such a statement then its objectivity has to be questioned.

The town of Milford did, briefly, serve as the base for a naval dockyard between 1797 and 1814 but it was too close to the mouth of the estuary and the land was privately owned. As a consequence the Navy Board of the Admiralty transferred its yard a few miles upstream to what soon became Pembroke Dock.

Lying on the southern shore of the Haven, the yards at Pembroke Dock were in existence for just over a hundred years. In that time they produced 263 warships and four royal yachts, becoming one of the finest dockyards in the world. When they closed in 1926 it caused widespread unemployment in the area.

The RAF created a flying boat base in the old dockyard in the years after 1930 and for many locals and visitors the sight of giant Sunderland flying boats on the waters of the Haven was a remarkable and welcome experience. When the base closed in 1959 it was a sad day for Pembroke Dock and for the Milford Haven waterway.

With the Suez Crisis of the early 1950s and the loss of the Suez Canal as a trade route - temporarily, as it turned out - Milford Haven again assumed a strategical importance when the construction of large, deep water oil tankers - to bring oil from the Middle East around the tip of Africa - became an imperative. Such giant vessels needed a secure base and Milford Haven, with plenty of deep water, was the site chosen.

In 1960 the Esso oil company opened their refinery just outside the town of Milford, closely followed by other refineries and pumping stations such as BP, Regent (soon renamed Texaco), Gulf and Amoco.

Within a few short years both sides of the Haven were encrusted with the derricks, tanks and jetties of the oil industry. It seemed as if long-term prosperity had come again to the area. By 1970 Milford Haven was the leading oil port in Britain, the second largest in Europe.

It was a brief flourish, however, as the "oil boom" finally stuttered to an end in the late 1970s and early 1980s. Esso closed down in 1983 after just 20 years of operation. Gulf lasted a little longer before finally shutting down in December 1997. The BP pumping station at Popton on the south shore of the Haven also soon closed.

Nowadays the Haven is still in use as a base for the leisure industry. Milford docks function as a marina and there are other centres for watersports at Neyland and Pembroke Dock. The Irish ferry operates out of the old dockyard at Pembroke Dock and the sight of ships, small and large, sailing up and down the Haven remains relatively common.

As Nelson may or may not have said, the Milford Haven waterway is one of the finest natural harbours in the world. It is an essential destination for anyone who wishes to see and experience all of Wales, not just the industrial heritage of the south east.

Red Letter Day, the series that looks at key events in Wales through archive footage, explores how the Suez crisis in the 1950s led to the industrial resurgence of Milford Haven. It's on Â鶹Éç One Wales on Tuesday 19 April at 10.35pm.