There is a DC long distance line from the Dalles, Oregon to LA that has been around since the early 70's. Its been upgraded a few times, and now runs 3GW of power over it. its very distinctive from the other lines nearby, since it only has 2 wires:
The grounding loops are very impressive. 6 mile loop of buried cable at either end.
It's generally understood now that using the earth for transmitting electrical current is a bad idea... It causes more corrosion in building foundations even hundreds of kilometers away, and soil microbes that navigate with electric fields die.
Therefore, most new DC transmission systems have a balanced pair of cables, and only use earth return for emergencies
The article says that the intertie has one +500Kv line and one -500Kv line. So why does current need to travel via earth? I would think the DC current flows in a loop via the two conductor lines.
The grounding loops are very impressive. 6 mile loop of buried cable at either end.
https://en.wikipedia.org/wiki/Pacific_DC_Intertie