Museums are broadening their program beyond the physical institutions by providing digital collections online. In digital collections, objects are prepared and presented particularly for the Web and the ambition is to provide the entirety of a physical collection. To make these rich and comprehensive data sets accessible, an explore mode is increasingly offered. The present study considers this mode, first by making sense of the term "exploration" and suggesting four functional principles in support of exploration in digital collections - view, movement, contextualization, and participation. On this basis, we compare eight well-known museums with regard to the explore modes for their digital collections. We have devised a three-part methodology, reverse information architecture, to address the question: How is the function of exploration manifested in the structure and interface elements of digital collections? With this unique method we use the given content to investigate how far the four principles are implemented in explore modes of digital collections and, broadly said, how explorable they are. The introduced approach to studying digital collections could be opened up to other fields to analyze a variety of Web interfaces in general.