At the foot of Monte Urgull, which closes the bay of San Sebastian to the north, stands the San Telmo Museum. Set between the densely developed old city and the lush green hill, the museum extension accentuates this boundary, while at the same time mediating between the urban realm and the natural environment. The shimmering, linear metallic structure, which fans out on the plaza, resembles an artificial rock face with natural vegetation. The new building complements the existing museum complex – a former Dominican monastery dating from the 16th century with an annex erected in the 1930s. The extension contains a new entrance lobby, a hall for various events, an exhibition space, a cafe and a library. This narrow, 150-metre-long volume was inserted in the tight residual area between the monastery and the hillside. The rock face was removed in part, and the outer wall functions as a retaining wall. In contrast to the sandstone facades of the existing buildings, the front face of the new structure consists of a homogeneous clip-on skin of perforated cast-aluminium panels that unites the various functions and visually reduces the overall volume. The skin is partly overgrown, with mosses and ferns that thrive on Monte Urgull sprouting from the openings. Over the areas of glazing, the perforated panels function as a light filter, while affording glimpses of the plaza from the interior.