In 2003 the Itoiz dam in Navarre, Spain, was completed and its reservoir filled in, submerging seven villages and three nature reserves in the process. Environmental group Solidari@s con Itoiz fought hard against this destructive ecological event. In addition to direct action including physically sabotaging the dam compound, the activists also closely monitored and filmed its construction. Created in clandestine circumstances, these hazy recordings emerge like a warning from the past in Maddi Barber’s spectral, enigmatic documentary.

Interviews with members of Solidari@s con Itoiz, shot in intimate closeup, ache with a longing for places that no longer exist. Their vivid recollections are juxtaposed with present-day footage of the area, which is now devoid of human presence; all that remains are lonely tree trunks standing in deep pools of water. In this record of erasure and displacement, the activists’ tapes are a kind of historical witness – of the military oppression of protestors, the demolition of local houses, and more. In one clip an unseen activist asks her associate to take in the sight of a beautiful rainbow – this fleeting natural phenomenon paralleling the beauty of the landscape, itself shortly to vanish.

For those unfamiliar with the issues around the Itoiz dam, it is difficult to grasp the full sociopolitical context of the activists’ work. Little information is provided about the reservoir’s creation, and the origins of the guerrilla footage are only made clear during the end credits. The film’s emotional impact, however, is palapable. By opening with a school play in which the children promise to protect the nation’s lands and rivers, Land Underwater hints at how younger generations are being failed by those in power.

• Land Underwater is released on 19 January on True Story