Mystery hole in the floor

Hi all,

I'm currently retiling the bathroom. Well, that's the plan, I've just finished removing most of the old tiles. The job has been put on pause, because I've come across a couple of noggin scratchers.

Exhibit A: The mysterious hole.
http://i69.photobucket.com/albums/i7...sc/br_hole.jpg
Apologies for the bad photo, I can try to take some better ones if it'd help.
This hole sort of appeared as I was taking up the last couple of floor tiles. It crumbled wider quite easily. Sticking my hand in, I can poke my fingers away from the wall a good 5~10cm. It tapers away from the opening, but there's still more space from the feel of it. At the bottom is what feels like nice flat concrete, which is approx 10cm from that base to the base of where the old tiles used to be. There's also a fair bit of sand in there, which from other bits isn't too surprising...

Any ideas? The house is an off-the-plan house built in 1970, reckon it could be a quirky risen floor?
Moreso, what could I do to fix it? Mix up some concrete and pour it in?


Exhibit B: Crap concrete
http://i69.photobucket.com/albums/i7...c/br_floor.jpg
Again, sorry about the photo.
In a few spots, eg the hole above mentioned, and at the edge of the door, the floor underneath the tile adhesive is pretty rubbish. In that there's been a few spots where it's just been a chunk of yellow sand with a light trace of concrete in it. Couple this with the mystery gap beneath the floor, and I'm a little worried. After I clear up the old tile adhesive, should I get rid of any and all soft spots? Or can I bang on the floor leveller and pray?

If I do have to take away more of the floor than intended, what do I do to build it up again? Can I just mix up some concrete and pour it in, then resume when everything has set, or do I have to do something more elaborate?


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