Pumped-storage hydro is the most efficient large-scale energy storage mechanism. 70-80% round-trip efficiency.

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

I stand corrected

Pumped hydro has an efficiency of around 80%, which is similar to lithium ion batteries. They also have massive capacity. The downside is that there aren't many places where they can be built.

What if we dammed the Mediterranean?

Funny enough that idea was proposed in the 1920s https://en.wikipedia.org/wiki/Atlantropa

Very.

However, nobody told the sun that, so it keeps heating up the seas, evaporating their water, and letting the resulting moisture drift until it gets pushed up into cold air by some land obstruction like a mountain range...

"Pumping" the water uphill in this manner is very inefficient, but like all legacy technologies, we're stuck with it until we can invent a replacement ;-)

While not quite as big as other pumped storage systems this is one system that is looking to avoid the exposed area and dependence on natural reservoirs: https://heindl-energy.com/