What is down the hatch in "Lost"?

[Atrahasis]

First, here is a summary of things I'm wondering about:

The "monster" is like a security guard....but what is it protecting? Something from getting near the hatch or something from escaping it...or does it really have anything at all to do with the hatch? It seems it's never around when people are even blowing up the hatch with dynamite.

What the heck was a man o'war doing smack in the middle of the island?

Who made the hatch and why?

Here are some predictions:

There'll be at least one new character introduced...he's been living under the hatch all this time.

How do we tie in fate, those numbers, the monster, the hatch, and scientific explanation all into one?

Some kind of psychic phenomenon is involved for sure. Maybe somebody was conducting psi experiments during one of the wars and someone with a lot of psi potential was left buried down the hatch.

[Atrahasis]

If "someone" is down the hatch, how have they been getting their air and water and sunlight, which you would need to grow basic food? Especially the sunlight...

There is the matter of the mysterious electric cable coming from the beach that the French woman has been using. Maybe whoever is in the hatch has been using electric bulbs to grow plants for food, a basic kind of hydroponics. But how do you make bulbs last that long unless you make your own or have a supply?

And what about air? How does anyone down the hatch get any?