There, after expenditure of nearly pounds 1m, the National Trust has brought back to life a site on which water power has been exploited for more than 400 years.
Its chief glory is the new water-wheel, 28ft in diameter and looking every inch a Victorian artefact, with its thick black rim and bicycle-type spokes, turning majestically to generate electricity; but the gorge through which the River Dulais flows is fascinating in itself, and full of historical associations.
On the road to Aberdulais I kept thinking of the mighty falls in the Yosemite National Park in California: terrific cascades, fed in summer by melting snows from the high sierras, tumble in vertical white walls for hundreds of feet. In particular, I remembered a notice at the head of one of them: 'If you go over the fall, you will die.'
Nothing so violent lies in wait at Aberdulais. The gorge, the river and the falls are all quite small - and yet, far from feeling disappointed, I found the intimate scale of the place most satisfactory.
The river, which rises in the Brecon Beacons to the north, is only 13 miles long and never more than about 20 yards wide.
Having no lake or reservoir along its length, it fills and drains very quickly. Yet at Aberdulais it flows over falls in a manner of which humans can easily take advantage.
The gorge itself is spectacular: during the last ice age, from 20,000 BC to 10,000 BC, glaciers ground lovely, rounded hollows and caverns out of the Pennant sandstone that forms the river's bed; and the water itself has a blue-green tinge, apparently a reflection from the ivy and scrubby rhododendrons growing on the walls.
Here, by the late 16th century, copper smelting had begun. Water was used to quench the roasted ore - a process that released clouds of sulphurous fumes into the air.
By 1631, the smelting had given way to a water-driven corn mill, and this, or its successors, evidently lasted right through until the early 19th century, for the gorge became famous as a beauty spot, attracting many artists, most notably Turner, who painted it in about 1796.
After that halcyon period came another wave of industrialisation in the form of a tin-plate works, with rollers driven by a water-wheel, which flourished from 1832.
Contemporary records make it clear that this turned the place into an inferno of noise and pollution.
Writing The Book of South Wales in 1861, Mr and Mrs S C Hall dismissed Aberdulais and Melancourt - 'two of the most famous cascades' - as no longer worth a detour.
'The tourist need not pause to visit them now, for the iron lords have ruined their picturesque.'
Yet no document speaks more evocatively than the record of David Williams who, as an eight-year-old in 1842, was working at Aberdulais as a plate carrier, 12 hours a day, seven days a week: 'Mother washes my face after work. Sometimes I wash my feet . . . Runs home to meals. Stays quarter of an hour to each . . . Never got burnt. Gets toes cut sometimes. Never been to school.'
Today, ruins of the tin-plate works still dot the floor of the site: pathetic reminders of the hellish, claustrophobic conditions in which people had to labour deep within the rock walls.
The enterprise closed in the late 1880s, and the gorge stood derelict until the National Trust bought it in 1981. Since then a marvellous transformation has been wrought, not least through the drive of the present administrator, Rick Pool, an Australian marine engineer and archaeologist who arrived in 1982 expecting to do six months' excavations, and has stayed ever since.
The old water-wheel had disappeared, and although the trust advertised for information about it, no details could be recovered.
The new wheel, which is the largest of its kind in Europe and began operating last February, was designed in Swansea by students at the University of Wales. Turning in the same masonry pit as its predecessor, it generates 25kW per hour - enough power for all the buildings on the site - and is the most spectacular of many innovations.
There is also an underground turbine, a hydraulic lift for disabled visitors, and a fish-pass that allows migrating salmon and sea trout access to the river's higher reaches - all neatly fitted into the bed of the gorge.
Small as it is, the Dulais packs a surprising punch. Even in normal weather, 30,000 tons of water pour over the fall every day, and in floods the amount can rise to a million tons.
In the torrential rains just before Christmas, some heavy object, probably a tree-trunk, hit the top of the weir with such force that all 13 cast-iron capping plates, each weighing a ton and a half, were torn off and flung down the waterfall.
One plate has since been spotted in a pool, but the rest have disappeared.
Although the bulk of the river goes over the weir unimpeded, the supply to the machinery is carefully controlled. Water for the wheel, for instance, passes through a stilling chamber and along a wide wooden trough before pouring into the 72 buckets around the rim.
The fish-pass is particularly ingenious. Every two hours a lock gate closes at the bottom, and any fish that has reached that point is trapped in an enclosure called a Borland chamber. This is filled with water pouring down a wide pipe. As the tube itself fills, the fish can eventually swim up it to a higher pool, and thence into a header tank, then up to the highest level - a climb in all of nearly 30ft. After an hour, the bottom gate is reopened and the cycle begins again.
As the fall had presented salmon with an unscalable barrier for at least 10,000 years, this is an epoch-making innovation.
Sizeable fish have been spotted through the observation window, ascending the pipe, and they have a good chance of spawning upriver.
The turbine, drumming away out of sight, is the least visible yet most valuable of the new installations: its output of 200kW per hour is sold to the national grid and, provide enough rain continues to fall, it may eventually help the site to become self-financing.
The other main source of income is visitors: in the first year of its reincarnation Aberdulais drew 30,000, but the National Trust hopes to double that number as the fame of the place spreads.
The site has already had one official opening - by the Prince of Wales - in 1982.
Now another is planned for later this year, provided the damage to the weir can be made good in time; and if on the day the weather happens to be bad, nobody will mind too much, for rain will put the Dulais in good shape and ensure that all the trust's fine toys are performing sweetly.Reuse content