There is a small crack extending 10 feet down the center of the slab floor of my garage. The floor sounds hollow when I hit it with a 2x4. Also, there is a 4" water drain and I can feel air when I put my fingers under the slab. It seems the soil under the slab floor has eroded away. There is a high water table in my neighborhood. Most of my neighbors get water in their basements, but I do not (yet?) The crack does not seem to have gotten worse over the 5 years I've lived here. There are bedrooms over the garage. What, if anything should I do about the floor of my garage? Thank you.
It doesn't look to me like the slab has shifted, since the crack appears level and isn't terribly wide. I would contact a concrete contractor, one that specializes in repairs, if there is erosion under the slab they can usually pump a slurry under it to fill in the void. Is your garage floor "Floating"? Look around the edges, if the floor goes under the walls, it's part of the support of the building and you want to consult a professional immediately. If the floor butts up against the wall, it's a "floating" floor and isn't part of the structure, however you may still have erosion under the wall footing, and I would still consult a professional, however a floating floor will be easier for a contractor to break up, install proper drains and such so it won't erode underneath again, and re-pour.
You could get an endoscopic USB camera from Amazon for about $20 that you could probably snake under the slab and inspect it, but I would consult a professional. Most of these types of things are cheap to fix now, but could end up costing thousands to fix later.