I had three cracked floor tiles in the shower and just removed them thinking I'd just simply replace and grout. However, after removing I noticed a lot of moisture and as you can see from the picture, there is a crack underneath exactly where the tiles were cracked. The shower hasn't been used for two days and if I dry this area it becomes saturated again within minutes, with water seeping up through this crack. Do I need to keep going down further? I don't believe the liner is torn as I have no leaks below. The home is 7 yrs old.
If the liner is good you don't need to go down more, though if you replace with the same size tile it may crack in the same place if there's a (evidently) structural issue there.
I'd be inclined to fill the hole with an "accent stripe" of 1" tile so the crack (when it re-cracks, as seems likely) can follow a grout line rather than crack the face of a tile.
You may have a leak under your slab, or in the wall. You said you hadn't used that shower for a couple days, so water isn't coming from there. If you dry the area, and within minutes it is saturated, water is coming in from somewhere. Constantly saturated concrete will eventually fail. Is that the same wall the shower fixtures are on? Is there a sink on the other side of that wall? Find out where the water is coming from, and fix that issue. Then seal the subfloor with epoxy, and re-tile.
Do you have an on slab foundation or a perimeter foundation.
If you have a crawl space and you probably have perimeter foundation but if you can not get under your house at all then you probably have it on slab foundation.
If you have a perimeter foundation you could try cheering up underneath your shower but even this could have movement and then if you did you would have a floated 2 inch thick and a waterproof membrane in your shower pan. And the water that you see sleeping around is just like a swimming pool in the waterproof membrane it won't affect anything outside of that and your walls won't rot out .
If you have an on slab foundation there's not much you can do from settling but you have a crack in your foundation and that way it crack your tile. to stop this you can try what is called a TS membrane or fracture membrane in any case so you won't have to use an accent strip.
I hope this information was helpful and a little more careful on what the other people that said
As noted previously, cracks in the mortar bed come from movement. What I would recommend, is replacing the showerpan with either a new mortar bed/ pvc liner, or to use (my personal preference) a Schluter Systems shower pan, or kit. It is easier to install, and has the correct slope to the drain, and carries less weight on the subfloor. It will also prevent all moisture in the shower area from going anywhere other than the drain by it's design. I have installed many myself, and have never been called back for repairs after. All you will need is a
1/4 x 1/4 square notched trowel and thinset to set before re-installing the drain.
You have discovered the fundamental principle of building things: stuff must be flat and level.
The problem is that the subfloor is not flat. You need to jack up the subfloor and add a truss so that it is perfectly flat. There are a lot of hacks out there who think the solution to the problem is shimming, which is a mistake.