Problem with Tub

I recently purchased a house and was doing some remodeling in the bathroom. I have a fiberglass tub that at first felt a little wobbly on the bottom because it felt like there was little support under the tub and between the subfloor. I recaulked the tub recently, but had noticed before then that it felt like a little bit of water was squishing around under the tub. Well, in the last couple of weeks it has gotten worse, and I can stand in the tub and hear water sloshing water back and forth as I step back and forth. The weird thing is that I checked in the basement and there is no water leaking below, it's bone dry. I drilled a small hole just through the subfloor directly under the tub, but no water dripped down. I do not have an access panel to look inside, and my only option would be to cut through my bedroom wall to look under. Is this a problem you have heard of, can it be fixed, or should I replace the whole tub? Help....any suggestions would be greatly appreciated!

Squishing Sound

For a novice like me, it sounds like you have a fiberglass tub liner with water between the old tub and liner. But, who knows.

I believe Steve's answer is correct. Somewhere in the tub there is a small leak that is allowing a very small amount of water to get into the chamber between the two layers of fiberglass. The bottom piece is watertight so the water does not escape which is good because at least the leak is not causing damage to the floor. I don't have a clue how to find the leak except to look very carefully around the drain. If you can find it, you may be able to seal it with something like epoxy. You can drain the water from the chamber by drilling a hole the size of a drinking straw and funnel the water out into a bucket. The seal that hole with epoxy.

If you unscrewed the drain, you'd have a path for the water between the layers to drain, then reseal it. It won't resolve the problem, but you might have a bad caulk joint somewhere between the walls and the tub that is letting water get under it.