Warning! Potential spoilers ahead.
“In Star Wars: The Last Jedi one of the new worlds is code named or possibly even called “Crait.” The world appears to be a giant salt flat with stretches of large black volcanic rocks. The black rocks look red because there is smoldering lava seeping out of them. It should be noted the setting isn’t like Mustafar from Revenge of the Sith with free-flowing lava. This is why early reports described it as “Mars with snow on it.” The lava is seeping out of the giant rock formations found scattered around the salt flat valleys in between the hot rocky terrain. It still isn’t clear if the white ground is salt or a light dusting of snow, but the white powdery substance collects around the banks of the settlement there and depending on who you talk to it’s snow or salt.
What is significant about the world? In The Last Jedi there is a mine there. The mine from what I’ve been told was used by the rebels as a secret base and source of funds during the fight against the Empire. During the Resistance era the mine is still producing some type of gem that is used to fund the Resistance. It sounds like it doubles as a way to fund the Resistance and act as a new hidden base for the Resistance after the events of The Force Awakens. It isn’t clear how much of that is pertinent to The Last Jedi but that’s what the rumors were from the set. However, the Resistance having a mine on “Crete” has been confirmed to us more than once and is a story element we’re confident in.
This is the setting in which the new First Order Heavy Assault Walkers (rumored to be called AT-4Xs as well) attack the Resistance mine. There is a visual effects shot of about ten of the walkers all walking shoulder to shoulder looking very apeish as they approach the mine. The Heavy Assault Walkers attack the mine in The Last Jedi and that’s their big action sequence in the movie. The B-wings we saw out in Bolivia attack the walkers. We heard there’s a shot in the film where some of the gems from the mine are dropped onto some of the walkers by the Resistance ships and they topple some of the walkers. […]
I have been contacted by a few sources to let me know the spelling for the new planet is “Crait.” I was also told to check page 170 of Star Wars: Rogue One: The Ultimate Visual Guide. So I did.
Lieutenant Heff Tobber had been stationed as a transport pilot at the rebels’ Crait outpost prior to its abandonment.”
Read more at Making Star Wars.