This door can be found in the rubicon wastes or the spawn area for Cerberus Vae 3.
It looks like a massive drill or door but what is behind it?! A vex incursion? A cabal base?
Randal the vandal? Will Wheaton?
What's even more interesting is that the fallen show up in the same area. Bungie wouldn't put scavengers next to a door that's never been revealed to us. Maybe it's hinting at something?
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I thought it was just a big drill thing which is what digs the cabal bases and tunnel networks underground. The size of the drill face matches the size of the huge circular doors. So I don't think there's any thing behind it other than a cabal (or crew of cabal) who drive that thing or more tunnels.