The Secret Sign Drymen, nr Glasgow
Many an artistic offering has promised to take its audience on "a journey into the unknown"; few can ever have delivered so literally. Not so much theatre as "environmental animation", this latest creation from the NVA organisation - the specialists in large-scale site-specific work formerly known as Test Department - takes you first on a half-hour bus ride out of central Glasgow, into the lush, vernal countryside between there and Stirling, kits you out in hard hat and waders, then takes you to the the river. And into the river, wading up to thigh-deep along Finnich Glen, a hidden gorge cut deep into the overhanging rocks, leading towards a stunning natural amphitheatre facing the Devil's Pulpit, a mossy mound protruding from the water on which, according to local legend, only Satan himself can alight when the river is in spate.

Before reaching the water, however, there's an initial stop at the "wishing tree", where we were invited to tie our own wishes or messages and read what others had left. This exercise inevitably took on faint last-request overtones, though the actual offerings ranged from numerous pleas for peace, love and understanding to "never let your friends down in a firefight". Thankfully it didn't come to that - actually the worst danger you're ever in en route, despite the guides' deliberately talked-up warnings, is that of stumbling in the water and getting a soaking, but it sure as hell feels spooky, images from horror and adventure movies flickering through your mind as you approach a concealed bend in the gorge. Sphinx-like rock formations loom out overhead from the gathering twilight, while strange plays of coloured light through the surrounding foliage signal the performers' covert presence. It's a place redolent with secret history, with tales of witches' gatherings, Covenanters' trysts and necromantic rituals echoing down the ages, and its primally potent out-of-time grandeur soon worked its way beneath this townie's skin.

The actual finale to the excursion, once we were gathered before the Pulpit and restored with hot toddies all round - definitely an inspired touch - proved to be a full-scale son et lumiere affair, transforming the site into an enchanted glade one minute, aglow with lambent golds and pinks, echoing with birdsong; crashing us into alien darkness and menace the next, only the very tops of the trees above us bathed in poisonous green light - rather like being trapped at the bottom of a huge well, with something wicked coming right your way. The climax to the show spectacularly orchestrated water (not to mention falconry) into this magical moving tableau of sound, light and landscape, but I'll leave the details of that surprise unspoilt: suffice to say there was barely an un-dropped jaw in the house.

As director Angus Farquar has himself acknowledged, the real star of the show is the gorge itself, but this is not to detract remotely from the production's tremendous technical and imaginative achievements. Being churlishly niggly, the whole experience did seem a trifle short, given all the build-up involved - though seemingly this was partly down to an over-hasty novice lighting operator on this particular night - but for all-enveloping sensory impact and sheer otherworldly beauty it'll be hard to forget.

Until May 16. Buses depart from the Tron Theatre, Glasgow. Bookings: 0141-552 4267.