They are only visible in traceroutes. Ripe seems to operate devices to collect BGP stats that also do periodic traceroute. Some of the packets they sent out crossed a router with IPs in the 240/4 block, but those are only used internally and not advertised outside the AS (apart from the suspicion of different Amazon AS advertising their 240/4 addresses to each other).
RIPE have been running Internet observatories for decades. I remember the members of the TTM (test-traffic measurement) project in the late 90s trying to get ISPs to install GPS receivers on the DC roof for accurate one-way latency observations.
The actual probes are tiny consumer grade routers (https://www.google.com/search?q=ripe+atlas+probe&tbm=isch) with a new firmware. RIPE provides these free of charge to volunteers (people and companies) who then run them in their network (e.g. homes and datacenters). (Of course larger form factors are available to volunteers with more in-demand locations.)
In return, volunteers get rewarded for uptime of their probes with credits that can be used to request custom measurements from the Atlas network.
There are probes _everywhere_; not just in terms of geography (https://atlas.ripe.net/results/maps/) but also in network locations that you cannot get access to via any other means; there really is no other service that comes close in terms of reach for global internet status testing.
RIPE Atlas is an amazing project. I remember getting 3 probes at a NANOG and telling my coworkers about it. They couldn't believe that it was a free project, and that I didn't have to pay anything for the probes. Great stuff from RIPE, for free! (Anchors cost dough but probes were free)
