The title of Richard Serra’s massive work in the Minneapolis Sculpture Garden succinctly describes the elements of its construction: two poles along the ground prop up five enormous flat plates of Cor-Ten steel. It is up to the viewer, circling around this essentially two-sided work, to discover the precarious balance of its forms (which seem to defy gravity but, in fact, are perfectly stable) and the dynamic interplay of line, space, and silhouette that are created by its composition. Serra has been exploring the properties of mass and gravity in sculpture since the late 1960s. In an early work in the Walker’s own collection, for example, a 60-inch-square sheet of lead is designed to be held flat against the wall and three feet off the ground solely by means of a lead pole that leans against it. Like the best Minimalist art, Serra’s sculptures, in spite of their seeming austerity, engage the viewer in intimate acts of discovery.