An unusual nocturnal performance is taking place nightly at the Edinburgh International Festival, as streams of runners wearing light suits turn Arthur's Seat into a work of illuminated, moving landscape art. Or perhaps it's better described as mountain-based mass choreography? Whatever you call it, it's pretty cool.

Speed of Light is the work of Glasgow-based environmental arts company NVA, previously known (among other things) for turning The Storr into a giant son et lumiere experience. This took an audience equipped with headlamps, guides and walking sticks on a midnight hillwalk through one of Scotland's most dramatic and inspiring landscapes. The massed pinnacles and buttresses were illuminated, while eerie 'soundscapes' drifted down from the ridges above.

In the Edinburgh event, on now, the audience are getting to be part of the show.

One of Scotland's major contributions to the Cultural Olympiad, Speed of Light fuses sport and art, say organisers.

During the nightly performances over the three weeks of the Edinburgh International Festival, which opened on Thursday 9 August, runners who have signed up from around the world wear the purpose-designed light suits, dynamically illuminating the hillside.

Contributing to the spectacular patterns of light across the hill, the audience carry specially commissioned lit walking staffs as they walk to the summit.

Innovative software contained in the walker's sticks also helps to create the musical soundscape. Resonance Radio Orchestra, a group devoted to the live performance of radiophonic art, has been commissioned by NVA to create the soundtrack for the audiences' experience on the hill.

Each light stick contains an individually encoded micro-computer, onto which the score is uploaded. Movement and altitude trigger the music, causing the pitch and volume to alter. As the walkers ascend Arthur's Seat they create a layered and ever-changing musical effect which will never be the same twice.

The walkers are guided across Arthur's Seat by walk leaders. Describing her experience at a recent rehearsal, walk leader Angelika Kroeger, who is travelling from Inverness to take part in the event, said:

'At a recent training session we had the opportunity to get a glimpse of how the event will look for the audience and it surpassed all expectations. It is fascinating to watch especially against the background of the more static lights of the city. It is like watching the sea, where you can't take your eyes off it, because it is constantly changing, and through your own movement you are part of this, of the choreography and the artwork.'