## 4 Discussion
Multi-temporal tracking is, undoubtedly, the key functionality that Sentinel-2 provides for hotspot monitoring, thanks to the combined high revisiting time of the two satellites. Our test on Apamea not only contributes to updating the information on the condition of this archaeological site as the last records date back to June 2016 (June, 2016), but more importantly, it demonstrates that the Sentinel-2 constellation allows for monitoring looting and similar anthropogenic activities at this scale. While single small holes can be resolved only by means of VHR satellite imagery, the experiment on Apamea suggests that we do not need exclusively VHR images to track new and expanding looting with temporal accuracy. Far from stating that Sentinel-2 or other HR optical space missions should and could be used as a replacement for VHR Earth observation solutions, we instead propose the joint use of these systems making the best of their respective advantages (that is, freely accessible regular acquisitions on one side; higher spatial resolution on the other). In Cyrene, we also proved that dense cloud-free Sentinel-2 time series can allow changes affecting cultural landscapes to be monitored and mapped with similar spatial accuracy and temporal granularity that can be achieved with an ad hoc VHR time series. Cyrene was a fortunate case study owing to the wealth of VHR images available via Google Earth. However, in cases in which the temporal coverage is discontinuous or cases in which such a dense VHR is not available, regularly acquired Sentinel-2 images become an asset for WHS condition monitoring. While supervised classification with broad classes (for example, 'urban' versus 'undeveloped land') is an agile mapping approach, our test of feature extraction proves that an insightful analysis can be carried out with Sentinel-2 data to geospatially assess the changing degree of exposure of vulnerable heritage. With this method, it was clear that not all the areas of the cultural landscape surrounding the WHS of Cyrene were vulnerable at the same time. Prior to 2014, the urban sprawl was predominant east of Shahat and could potentially impact only the east necroolis. Starting from 2014 it appears that the urban sprawl became an increasing threat also for the conservation of the south necroolis, west of the modern town. Our experiment suggests that, if a dynamic assessment was undertaken in near real-time as new Sentinel-2 images were ingested in the time series analysis, the satellite-based condition assessment may inform the decision-making process and help to monitor the compliance of local land use changes with planning and landscape policies and regulations. As found in Cyrene, the distinctive features relating to land cover and land use changes may be local manifestations of more regional phenomena. These situations can be captured over wide regions by exploiting the large swath of single scenes or mosaics of multiple (but coeval) Sentinel-2 images. To simulate how Sentinel-2 could be used for this purpose, we ran the same test to extract and map block features due to the urban sprawl across Cyrenaica in Libya. We analysed a multi-temporal mosaic that we made by combining Sentinel-2 images available over Cyrenaica, from 29 July 2015 to 4 October 2015 (Figure 10a). Our analysis suggests that in 2015 a number of block features were already widespread and common features in all the major urban settlements of Cyrenaica. The comparison with the Google Earth VHR time series pre-dating the Sentinel-2 mosaic confirms that the number and density of these block patterns started increasing since 2013, according to a general trend that can be observed in many other locations. By intersecting the block features extracted from the Sentinel-2 mosaic and the location of known archaeological sites, three hotspots are found in the ancient Greek towns of Apollonia, Ptolemais, and Tocra (Figure 10a).