Cosmic microwave background sky thresholded at moderately positive (left) and negative (right) levels. For high thresholds, the excursion set is dominated by isolated components, while at low thresholds, it gives the appearance of a single connected surface indented by numerous holes. For sufficiently low thresholds, the holes fill up, and the excursion set covers the entire sphere, which is composed of a connected surface without a boundary that encloses a single void (cf. Fig. 1).