How tightly-sealed does the door need to be? Airtight? Watertight? A fit like a regular door?

A big spherical rock in a concave hole that acts like a cork on a wine bottle

+1 intriguing question.. Q: what's the technology level available to the makers of this door ?

Must that door be usable like a regular door (opened and closed frequently)? Or is a solution that can't be (easily) closed a second time acceptable?

How long did the door makers expect it needed to last?

Can you explain any difference between "the elements" and" tectonic and thermal shocks"? In either case, what kinds of seals have you already ruled out?

Judging by comments on the "gold" answer, you should specify if the door needs to be locked for a million years before people can open it, or if it is OK that the first person to come along can open it, because you don't expect looters before then.

I assume simply putting it on the moon doesn't count?

BTW at that long time nobody will use door anymore, not even "talk“ or "anybody" will exist or make sense

"The makers of the vault had access to future tech" - so why can you just not make the door out of technologium ?

A million years is a long time for anything man-made to survive (like a door, or the things inside the vault), or for any geologic feature to remain unchanged. In a million years only a nuclear scientist will be able to infer we existed.

The door does not need to be resealable once opened. It can be an open once door. A door that only needs to be opened once is called "a wall". Seriously, it's just a patch of wall with a load-bearing lintel above it. Knock it down when you need to get in, the end.

@Spratty It's an interesting idea. I've added the clarification that "it should be fairly obvious that it is meant to be opened". If you can solve that, this could make a good answer

@0x263A Interesting idea; want to flesh it out as an answer?

Advice concerning questions asking HOW to implement a technological procedure or device

@Spratty to that end: ignore the doors, what are the walls of that vault made of?

I wonder whether the vault (or its outer room) should remain accessible by at least "the right people" during that million years of wait, and if yes, whether they are allowed to grease the door to whatever inner stuff there is. And if not, how to ensure that the vault would be located past that mega-millenium.

The vault is located such that it is only accessible by the right people. -- What's the purpose of that inner door then?

@Hobbamok - I don't lay claim to the sort of materials/engineering to be able to answer that; I was simply pointing out that if something only needs to be opened once and doesn't need to be closable/reusable afterwards, it's not really a door as much a a section of wall you can knock down when required. Seriously, whatever you make that wall out of, make part of it with a lintel over the top and supports down the sides and knock it down when needed. What material lasts a million years? I believe there are millions of tons of igneous rocks lying around that are way older than that already.