This paper presents the design and field evaluation of Syros GeoPaths, a mobile app that has been developed to support visitors of the Syros GeoPark, so they learn about its geological significance while hiking in a protected area. Syros GeoPaths supports the user experience before, during, and after a hike following principles of calm design. Before the hike, the app informs hikers about the preparations needed and the characteristics of the paths. During the hike, the app helps users navigate the GeoPaths and points of interest while presenting geological information in visual and audio descriptions. After the hike, the app offers certificates of achievements to visitors, based on statistics of the user behavior during the hike. The app has been developed in open technologies: Android and OpenStreetMaps. It fully operates without requiring data connection, since this is sometimes unavailable in this area. A field evaluation of the Syros GeoPaths mobile app has identified several user experience issues, which are considered to improve this app and, more generally, to design hiking apps for learning about geology, archaeology, or other fields of interest.