When Martha was handed the Osterhagen Key in the last episode of Doctor Who, The Stolen Earth, we were left in no uncertainty that this was something not to be used at all costs. There were worried faces, stern warnings, and people who didn’t know a thing about it.

But what on earth is it?

Good question, and it’s one that we’ll try and unravel a little here. We should point out that we have no inside knowledge, nor have we sought out spoilers to try and answer the mystery. This is all speculation of ourselves and Den Of Geek readers…

The most obvious place to start is with an anagram. Osterhagen itself can be rearranged to form the words ‘Earth’s Gone’. So does the key bring it back? Or is it the kind of key that you use, as we suspect, as a very last resort once the fate of the world looks doomed? It wouldn’t surprise us if it proved to be a suicide device of sorts for whoever chose to use it, for use when the future of the planet looks doomed. Which, to be fair, it does.

If that’s the case, then what does the key actually do? Does it unlock something? The only lock that we’ve had mentioned in the last episode or so was the timelock on the Time War, that somehow Dalek Caan managed to navigate to bring back Davros, albeit at the cost of his mind. Could the key unlock the Time War, and just what would that open up? Would it mean the Time Lords return? Would it mean whoever used the key lost their mind in the same way that Dalek Caan has?

What hurts this theory is that UNIT, rather than Torchwood, have been holding the key. Who gave it to them, because it’s unlikely they invented the technology themselves? Could it be the Doctor? River Song?

So could it move the Earth? As was noted in The Stolen Earth, this has been done before, and it’s just been done again. Could the key be what’s needed to move the planet back, or would that be too easy?

Several people have reported that if you search for the Osterhagen Key online, you firstly get, er, a whole lot of websites speculating what it is. But what you also find is references to Burning Man (although we had no joy when we went searching). That could link back to The Master at the end of season three, but there may be a little grasping at straws there.

What’s interesting, incidentally, is that Harriet Jones was very keen for the key not to be used. Having rewatched The Stolen Earth, we get a very obvious shot of the Dalek re-aiming before appearing to shoot Harriet. We strongly suspect that we’ve not seen the last of her, though, and that she’s aligned herself somehow (by choice or otherwise) with the Daleks. Thus, the Dalek re-aiming was potentially to shoot the computer and screen rather than Harriet Jones herself.

If that is the case, then she knows the impact that the key can have: does that mean it’s the ultimate weapon against what’s been planned? Or, as Prime Minister, was she something to do with acquiring it in the first place through her alien encounters?

Further theories? There’s the ongoing fear that it’s the dreaded reset switch. Surely they’re not going to use it to wind time back again or something like that? After last year, we suspect that won’t be the case this time round, but you wouldn’t totally wager against it.

Could it perhaps be a means to reopen a rift? Or is that rewinding to the end of series two?

And if you want something a little more outlandish, could it be some kind of ‘backup’ of the Doctor himself? He’s the person most likely to have given it to UNIT, after all, as we've already speculated.

All of which leaves us, well, precisely no further, short of a feeling that the aforementioned key is a crucial part of the Journey’s End mystery. Share your own theories in the comments box below…