Visitors to the Metropolitan Museum of Art’s Roof Garden this summer might suppose at first that the maintenance crew has been tearing up the terrace’s paving stones in search of a leak. Displaced slabs are stacked next to rectangular cavities exposing underlying dirt where puddles and rivulets have gathered. In fact, the apparent disarray belongs to an installation by the French Conceptualist Pierre Huyghe, an untitled work commissioned by the Met for its annual Roof Garden show.