Whenever i search for the physical location and path of WAN connectivity, i only find submarine cables, and a site that seems like a paid service. Is there no free map where i can e.g. show the route that the connection from, lets say, Prague to Lissabon takes? Im quite suprised that such a trivial thing seems to hard to find out. So my question is - why is there no map of the most relevant, large continental internet cables, while there is a map for submarine cables? Is there some sort of instance that overlooks the most important cables, like the TAT-14 commitee?
Trivial? I'm not so sure.
You'd have to ask many people at many service providers (and in turn their service providers, and then their (dark or lit) fibre providers and then their cabling companies) about many many details, most of which they won't, can't or must not make public.
You might be able to get some high-level diagrams (as in: a powerpointable JPG showing a country or half a continent with some dots and straight lines) from companies that dug the trenches and put the fibre in, or from companies offering lit or dark fibre in trenches someone else built for them.
Then again, they have little clue what the companies leasing the fibres actually do in terms of their underlying network topology, and what they (and their wholesale customers, reselling to other ISPs) do atop of that with the logical network topology.
From the outside, one only gets a blurry glance at the logical network topology of different ISPs from reverse DNS lookups on hop's IP addresses showing up in a traceroute, where ISPs sometimes encode site names or city names in the PTR record. The most accurate info in there might be that IP addresses show up which can be assigned to a well know internet exchange - which in turn might be spread across multiple datacenter sites around a single town with multi-kilometer distance.
Other than that, there might be geolocation databases of IP addresses and hints taken from the IP Registries in the Whois database - but again, that gives you just hints at where an intermediate network device might be. All those "graphical traceroute" tools are just based on information sources like these.