NATIONAL Theatre Wales's new production, Mametz, which runs on a sheep farm near Usk until Saturday, July 5, is a site-specific piece with a site so perfect for its subject - the battle for Mametz Wood in 1916, fought by the 38th (Welsh) division during the First Battle of the Somme - that for two and a half hours we felt we had been transported in both place and time.

Written by Owen Sheers (author of Resistance), inspired by his own poem Mametz Wood and drawing on the prize-winning poem In Parenthesis by David Jones, who would survive the conflict in which 4,000 of the division were killed and wounded. Jones was one of a number of Welsh and English war poets who would later draw on the bloody six-day battle for Mametz Wood - Robert Graves, Siegfried Sassoon and Llewellyn Wyn Griffith, whose memoir Up to Mametz is a keystone of the NTW production.

The ambition and invention of this production is hugely impressive, the use of the site imaginative and atmospheric - but it would be unfair to anyone planning to see Mametz to reveal the specifics of its cleverness - you'll know you're in for something extraordinary as you enter the site to the sound of the distant, horribly real, rumble of battle and watch as soldiers run through long grass with messages from the front. A great ensemble cast comprising actors as young as the teenagers they're playing serves to underline something we know we know but perhaps don't fully recognise - most were barely out of school and by this stage in the war, the height condition of 5'7" had been reduced to 5'3" ...

The battle for the single square mile of forest saw loss on a massive scale, and Mametz sharpens the focus to bring the lives behind the unthinkable numbers to the fore, movingly pressing home the point that all these men were fathers, sons, brothers, lovers, husbands ...

The site at Great Llancayo Upper Wood is stunningly beautiful, contrasting, like Mametz itself, with the ugliness of war, and transformed into a landscape of trenches and gunfire, and an estaminet where soldiers gained some relief from the onslaught, where we also got a glimpse of what the struggle for control of the wood did to live lives of the people to whom it properly belonged.

Thought-provoking, moving and unforgettable, staged just days before the anniversary of the battle for Mametz Wood, this production is a must-see. And do not be put off should it rain. By the final scene on Thursday evening, the heavens had opened over Great Llancayo, which only added to the atmosphere and the sense of sorrow, loss and waste before us.

Mametz is co-commissioned by National Theatre Wales and 14-18 NOW, WW1 Centenary Art Commissions, supported using public funding by the National Lottery through Arts Council England and the Heritage Lottery Fund.

Mametz runs until Saturday, July 5 at Great Llancayo Upper Wood near Usk, Monmouthshire. Book online at nationaltheatrewales.org