## 3 Results and Discussion
### The 2014 Surface Disturbance Event  
In its press release of 9 December 2014 [10], the Ministry of Culture in Peru reported about an event that occurred very early in the morning on 8 December 2014. Environmental activists accessed the area near one of the most famous geoglyphs of Nasca, known as the \"Hummingbird\" (_El Colibri_), and unfurled tall, yellow cloth letters under the bird's beak to compose a message [25].  
Photographic evidence collected by the Ministry in the following days [26; 27], as well as the photographs and videos published in the web media [28], clarified the type and extent of the damage observed. In particular, the in situ inspections highlighted that \"the route followed to access the geoglyph was irreparably marked, in addition to the disturbance of the surrounding terrain of the figure of the Hummingbird due to the movement and circulation of people\", the latter affecting a nearly rectangular area with an extent of around 1600 m\({}^{2}\)[29].  
For the purposes of this retrospective analysis, the above background information suggested that an event such as this could cause changes at the surface that could have been captured in the archive COSMO-SkyMed satellite SAR images and could be extracted to create a digital record. In this regard, corroboration can be found in the scientific results that were recently published in [17; 30]. These authors used a pair of pre- and post-event airborne L-band SAR VHR scenes (March 2013 vs March 2015) that were acquired from NASA's uninhabited aerial vehicle SAR (UAVSAR) platform, and they found highly decorrelated zones indicating apparent surface disturbance due to the December 2014 event.  
Figure 6a shows a zoom down onto the plateau where the Hummingbird is carved (see also Figure 2c), with a rough delineation of the area affected by the 2014 event as reported in [31], which is centred at approximately 14\({}^{\circ}\)41'30\"S, 75\({}^{\circ}\)08'56\"W and has an overall extent of around 2000 m\({}^{2}\).  
Figure 6b,c compares two InSAR coherence maps generated using one cross-event (10/07/2014-12/04/2015) and a very recent post-event (13/12/2017-30/01/2018) COSMO-SkyMed pair.  
In the post-event pair, the plateau shows generally very high coherence with \(\gamma\) values of 0.8-0.9 (Figure 6c), thus indicating the absence of recent surface disturbance in the area of the geoglyph. Only a few spots of coherence loss in the valleys around the plateau can be observed, and they are mainly due to the presence of topography and geometric decorrelation.  
Analysis of the pre- versus post-event scenes instead allows the identification of a noticeable decorrelation feature to the west and northwest of the Hummingbird geoglyph (Figure 6b). This seems to match very well with the area where the disturbance occurred, including both where the message by the activists was temporally installed on 8 December 2014 and the path that was delineated in [29; 31]. Despite the relatively small \(B_{p}\) of 14 m of the cross-event InSAR pair, the effect of decorrelation due  
Figure 6: (**a**) Google Earth overview of the plateau where the Hummingbird geoglyph is carved (VHR image © 2018 DigitalGlobe), with a sketch of the geoglyph and indication of the area affected by the 08/12/2014 incident as delineated in [31]; and (**b,c**) InSAR coherence \(\gamma\) for two COSMO-SkyMed StripMap pairs: (**b**) 10/07/2014-12/04/2015 (cross-event) and (**c**) 13/12/2017-30/01/2018 (post-event).  
to the temporal baseline of 275 days is much more apparent also in the valleys around the plateau, whereas coherence across the plateau is generally high (except in the area of the incident).  
The shape and extent of the decorrelated area indicating surface disturbance in Figure 6b is similar to that found via the UAVSAR VHR campaign presented in [(17; 30)].