I might misremember, but aren't some of the submerged sea power cables (Norway, Germany, UK) DC? If so, why? And how does that rhyme with AC being more efficient?

There’s a dedicated DC trunk line running down the entire west coast. It’s how hydropower generated in Washington state is sold to California. High voltage DC to bridge grids is really common.

https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie

I don't know if it is the same issue but AC lines longer than a certain distance will approach the wave length of an antena and start to radiate energy (like a radio broadcast antena). In that case you use DC. When I studied this back in school I was told there was such a line in South Africa (if I recall correctly).

Disclaimer that I'm not an expert. As far as I understand, AC runs on the outside of the cable rather than through the conductor. DC does not. That's one of several reasons why.

AC current crowds towards the surface of the conductor, called "skin effect". Imagine applying a color gradient fill to a circle. Increasing the frequency makes the gradient sharper near the surface and shallower near the center.

My understanding here is that the saving comes from needing fewer cables, although I’m sure there’s a lot more to it than that.

A whole bunch of different effects.

AC/DC have different costs for voltage converters and per-distance efficiency, so there are some distances where AC makes more sense and others where DC makes more sense; the distance changes as tech improves.

AC mostly conducts on the outer surface of the wire, while DC conducts with the whole cross section, giving you different scaling issues as the current changes.

Under water, AC suffers from significant capacitive loss — the wire acts as one side of a capacitor and the entire ocean as the other.

At certain frequencies and wire lengths you also get inductive losses, though IIRC that affects only RF cables in practice and and the design trans-Siberian railway in theory as no other place even seriously considered having a sufficiently long conductor for the frequency used.

As others mentioned, undersea cables suffer from capacitive losses. But another big factor is grid synchronisation: two AC grids cannot be joined if they are not perfectly synced. Their phase must match. DC having no such phase, it’s very useful to connect independent grids (e.g. UK to the rest of the EU, or the three main grids in the US)

AC is quite a lot more dangerous than DC for humans at ~hundreds of Volts. ElectroBOOM has made a nice demonstration of this: https://www.youtube.com/watch?v=snk3C4m44SY

DC arcs are much harder to extinguish. Switching 10A of 240v AC is trivial. Switching 10A of 240V DC is much more complicated in terms of switchgear. Sustained arcs are dangerous.

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.

https://en.wikipedia.org/wiki/Pacific_DC_Intertie

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.