Designing Kitchens and Bathrooms - Leak from bathtub??? Where's it coming from??

Here's the situation. This weekend after taking a shower I went in the basement to get some clothes and noticed a leak. It's actually coming from the rear of the bathtub. In the basement I can see all the bathroom plumbing and non of it's leaking. I inspected the entire tub for cracks and can't find anything. I also inpected and re-caulked the entire outside area of the tub.

It still continues to leak everytime we take a shower. Any idea where this leak is coming from?

Sometimes when you stand in the tub to take a shower, and the old plumber's putty under the drain lip has deteriorated, the weight of someone standing in the tub will depress the tub just enough to allow it to leak around the drain lip.
Try this.
Remove any strainer on the drain. There should be four crosspieces down in the drain. There is a tool for this, but you can use the handles of a pair of channel-lock pliers with a screwdriver in the teeth for leverage to stick down in the crosspieces to unscrew the drain counter-clockwise.
First, heat the drain lip with a hair dryer to loosen the old putty.
Clean off the old putty. Roll a roll of new plumber's putty in your hands about the size of a pencil and wrap it around the depression around the drain hole.
Tighten the drain back down on the new putty. Wipe away the excess.
See if that doesn't stop your leak.
Good Luck!
Mike

Does the problem happen when taking a bath?

If not, then it is probably not the drain.

Do you have tile surrounding your tub / shower?

In many cases the tile grout could be old and have small cracks. This will allow water to pass through. Re-Grout and seal the tile, this will more than likely solve your problem.

The problem is I can actually see the underside of the drain and all plumbing from my basement. There's no leaks coming from the area at all. The tub is a fiberglass tub. I believe it is two pieces. My father-in-law stopped by yesterday and is also mind boggled and he has been in constuction all his life. He thinks to could just need to be re-caulked. I'm gonna take care of this tonight and see if it leaks tomorrow at all. Thanks for the help.

If someone is using the shower and the curtains hang halfway out of unit, or slightly open.......the leak will follow the edge of unit and go into floor. Number one leak in this situation.