[varmint]

The answers to most of your questions will simply be "yes". It will be both above and below ground. The depth will vary depending on soil and ground water levels and a myriad of enviro entail laws.

It is a continuing job for thousands to keep a pipeline running. From the pump stations to cleaning pigs and security. Imbeciles like to shoot at pipelines for some reason.

It's all a huge undertaking that only makes sense when compared to transporting it by rail.