I have just invented the Macguffin. It is ultimate wondrous power for anyone who is wise enough to wield it...
But humanity isn't ready for it yet. I need to put it out of reach for a significant amount of time, such that it cannot be recovered 'soon' - but that it should be recoverable at some point.
To facilitate its recovery, I am going to write some prophecies - that a chosen one will discover, interpret and know when the 'vault' will open.
But ... how does my vault work?
I'm aiming for a significant amount of time out of reach (beyond my lifetime). And that I can reasonably accurately prophecy its return. It needs to cope with ... well, the worst that humanity might do. So I can't use anything simple like a clock, because I'm worried it might not survive.
It can be something periodic, or a one off event. If the latter, ideally it'll be such that it can be found at any point someone decodes the prophecy (but ideally not find it by accident). If periodic, ideally it'll be fairly easy to 'unlock' if you know you should, but also hard to do accidentally.
Assume a substantial amount of resources (anything that's at least theoretically possible using real world physics), but avoiding supernatural.
Aiming for something like the clock of ages in Tomb Raider, where a certain planetary alignment is needed - specifically trying to work out how my macguffin ends up as a plot device in a future story after I'm dead.