To detect significant network disconnections we monitor the status of Atlas probes. We essentially seek for synchronized disconnections of probes that are in the same geographical or topological area. See our research report for more details: A. Shah, R. Fontugne, E. Aben, C. Pelsser, R. Bush, Disco: Fast, Good, and Cheap Outage Detection. TMA 2017
Here is an example for AS35540 with two disconnections late in 2017.
The graph shows the disconnection level, which is a metric taking into account the number of disconnected probes for this AS and the synchronization of their disconnection time. Larger values represent stronger evidences of the network disconnection.
To avoid too many false positives, only values higher than 10 are reported. We also monitors only networks and countries that have at least 10 Atlas probes. If you click on the graph you will obtain the list of disconnected probes along with a visualization of traceroutes initiated by these probes (using Tracemon). This visualization allows one to see the outage from the inside and identify where the disconnection happened.