Drive northeast from Bilbao toward the coast and into the Urdaibai estuary — a UNESCO biosphere reserve that covers the river mouth, surrounding marshes, oak forests, and coastal dunes between the towns of Gernika and Mundaka.

Urdaibai is the green interior of the Basque Country made accessible without being managed into blandness. The estuary at its center is one of the most significant wetland habitats on the Atlantic coast of the Iberian Peninsula — a stopping point for migratory birds, a nursery for marine species, and a landscape that shifts between tidal flat, forest, and open water within a few kilometers.

The town of Mundaka sits at the mouth of the estuary where it meets the sea. Mundaka is known among surfers for a left-hand wave that breaks over a sandbar at the river mouth — considered one of the best river mouth waves in Europe. You do not need to surf it to appreciate it. Watch it from the harbor wall if the swell is running. The mechanics of the wave — the way it peels along the sandbar in a long, consistent line — is worth understanding even as a non-surfer.

Walk the estuary path from Mundaka toward Busturia if the morning is clear. The path runs along the water's edge through mixed woodland and opens onto viewpoints above the tidal flats. An hour out and back. No facilities, no signage beyond the path markers, no reason to rush.