Mario Merz in particular produced welded metal structures of impressive dimensions, creating, in 1968, his seminal series of iron igloos. Likewise, Marisa Merz used shiny, coiled strips of aluminium stapled together to form an enormous hanging ceiling sculpture. The concern with process and with the primacy of materials, both man-made and natural, led to 'interventions' with the landscape, giving rise to the land-based art of the 1970s. Giuseppe Penone, one of the early tree-huggers (who did actually embrace trees for his art), recorded his various actions and interventions in photographs.