In theory, you can underground any kind of power transmission line. However, in practice the cost per mile is at minimum four times greater than an equivalent overhead line. This cost discrepancy increases along with the voltage level. For example, Duke Energy quotes the cost of a 500 kV underground line being 10-25 times more expensive:
Underground lines fail very rarely. But when they do, the cost and time needed to repair one are far greater than for overhead.
For these reasons, utilities will only underground a high voltage line (69 kV and up) when the cost is justified in dense urban areas, very near an airport, or other such obstacles to a project.
Just because you cannot see an underground power line doesn’t mean that the permitting or environmental issues will be any easier. In fact, the environmental impacts of building underground power lines, when you how have to dig a new underground trench or tunnel, are often more invasive that building an overhead line. This is especially true going under rivers and streams. Building a high-voltage underground line is similar to building a buried pipeline, minus the oil spill risk.