The Lost Index: NATMUS is one of a series of locative iPhone apps produced by Emma Whittaker and James Brocklehurst. Sited within museums, the apps incorporate iBeacon technology, binaural sound recordings and perceptual illusions, in conjunction with real-world artefacts from the museums' collections, to create imaginary playable experiences. Forming part of research that investigates situated narrative experiences, the apps offer new narrative frameworks and an innovative approach to archive and heritage interpretation. NATMUS was installed in the Dieselhouse Museum and National Museum of Denmark in Copenhagen as part of the 8th International Conference on Interactive Digital Storytelling in November 2015.