Back when the telecom companies were running cross-country cabling in the 80's 90's in Canada, there's a large part of the country starting about central Canada going east-wards that's called the Canadian Shield. It's made of _EXTREMELY_ dense rock. Cable companies had to literally make a series of controlled explosions to run cables underground or to even set communications poles in the ground.
There's a lot of factors that go into running underground cable, heck even above-ground can be problematic too!
I believe Telus buries their cross-country fiber cables at least 18 feet underground, far below anybody even coming close to excavating it, due to the sheer amount of financial damage that could be caused by a fiber cut.