Tallest peak and lowest valley?

[Halfcell]

I dont know why people keep focusing on Kedge, it's in a lake. Granted BB is at sea level, but even still the assumption that a lake on the other side of a zone called Butcherblock Mountains is at or below sea level is dubious.

What we can be sure of is that Siren's grotto starts at sea level and goes down. Also, I dont know exactly where ST and DN are, but assuming they are below their respective teleporters, they are good candidates for lowest zone. Hell, the lava pit in NToV might be the lowest point in the game. I dont see any above ground temple there.

As for highest zone it is almost certainly High Keep, have you seen the drop down to EK on the back side of Highpass? If it's not high keep it's probably Frontier Mountains, some of those peaks are above the top of the skybox. Everfrost seems unlikely to me as I suspect it's cold because it's up north, not necessarily high in elevation.