The Urdaibai Biosphere Reserve is a nature reserve which was declared a site of great environmental interest by UNESCO in 1984. One of its most attractive areas is the estuary of the River Oka, in the very heart of the Reserve. It is the largest, best preserved natural area in the Autonomous Community of the Basque Country, measuring around 1000 hectares below the tide line from the sandbanks of Laida (Ibarrangelu) to the town of Gernika-Lumo.
Close to the town centre stands the starting point of a 14 km circular trail around the estuary, which crosses from one bank to the other via a pedestrian/cycle bridge and runs from Gernika-Lumo to Kortezubi and Forua, and to the Murueta Punte lookout point. From this trail, walkers and cyclists can see how various species of animals and birds cope with the falling and rising tides and with the different habitats that have resulted from the coexistence of natural landscapes and human use of the area.