I really quite like the idea of there being a mysterious door at the bottom of the Marianas trench, but the "door to hell" schtick is somewhat generic in my opinion. Not sure what, but I would try having something else behind the door.
Agree. Interesting concept, but door to hell is a bit easy. Also, just as a side note, both your D's call their handler Doc, which makes them sound like the same person. This isn't even touching on their identical reactions to the door itself. You might want to give your second person a different "voice".
Well, here's my ideas of what could be beyond the door:
1. It leads to a cave near the summit of Mount Everest.
2. Opening it makes you hear a song that will [DATA EXPUNGED].
3. Opening it leads even lower into the Earth; you see a small speck of light if you look into it.
4. Overwatch HQ.
The idea of it leading to the O5 headquarters actually amuses me greatly. Don't go off on my word alone, as I'm but a young'n here myself and probably don't have the best advice to give, but if the hatch just led to wherever the O5 are holed up at any given time it would be a hilariously dangerous item for the Foundation to have to protect. Even if it's in literally the most remote place possible, if it's a direct line to some of the most important people on the planet, they would have no choice but to guard it.
Not number 2. Never number 2.
After much thought I feel that I've come across a stumbling point, which makes what is behind the door extremely important - pressure. Considering how ridiculously high the pressure of the water is at the kind of depths you're talking about for this SCP, how can you open the door, let alone open it without obliterating the contents of said door? Unless the other side of the door is flooded with water at a very similar/the same pressure, the whole venture is pointless without some kind of wizardry.
If it's a trap door, it may open inward. It's obviously an anomalous door, so there's no particular reason why pressure must be an issue. Either, yes, it opens to someplace of equally high pressure, or the other side of the door is simply cut off in a way so that the water and pressure is incapable of passing through it.
Or you could just have the door opening cause a massive flood of super pressurized water and insane havok being wreaked until they somehow manage to close it back up.
Say, maybe the door could just lead to a random door anywhere in the world. If that were so, how would water from those depths behave if you put them instantaneously above sea level?
If not that, is there any way I could make it as Overwarch HQ? :D
Door to hell is definitely boring and overdone (and we kinda had one, 'cept it was so controversial that it was retconned out), and I can't help but think that having it in the Challenger Deep actually makes it much harder to hide, considering how much exploration is being done there now.
This will inevitably be compared to the Document Recovered from the Marianas Trench http://www.scp-wiki.net/document-recovered-from-the-marianas-trench which may make or break the piece when it comes to ratings.
As for the Overwatch HQ idea, the Overseers policy on never, ever making contact with any kind of SCP would end up leading to fifty million or so logistical nightmares. Firstly, how would there exist any kind of door or pathway within their HQ that was never checked and secured? And secondly, what would happen to the power structure of the Foundation and policies of the Overseers if they were all suddenly exposed to this SCP in their base location?
Technically, the Marianas Trench attracts a LOT of attention, and there is a metric crapton of probes/submarines/people that are stationed there. Even James Cameron went to the bottom.
You don't need to use the Marianas Trench, there's plenty of places on Earth.