Could my bathtub really be draining straight into the ground under my house?

Stop using that shower immediatly. Seven years of dumping hundreds of gallons of gray water directly into the soil under your slab is exactly why your drywall is cracking. You've essentially created a localized swamp that is compromising the load-bearing capacity of the soil. When dirt gets saturated like that, it either swells or loses its ability to support the weight of the footer, leading to the **subsidence** you are seeing. It is a massive structural red flag. The plumbing fix itself is actually the minor part of this nightmare. You are looking at installing a **P-trap** and connecting that tailpiece to the main drain line that was left capped. Whoever did the original rough-in clearly walked away before finishing the job. You will likely need to dig out some of that mud to get enough clearance to solvent weld the new fittings. If that main line is PVC, it is a basic cut-and-glue job, but doing it while lying in seven years of accumulated soap scum and skin cells is going to be miserable. You need to treat that crawlspace or sub-floor area as a biohazard zone. Even though it is just tub water, seven years of organic material sitting in dirt creates a breeding ground for bacteria and mold. Wear a high-quality respirator and some disposable coveralls before you spend any time down there. You will want to get a small utility pump to pull out any standing water and then run high-velocity fans for a week or more to dry the dirt out as much as possable. Once the leak is stopped and the ground is dry, you have to address the structural side. Don't just patch the drywall cracks and call it a day. You shold get a structural engineer to tell you if the foundation has stabilized or if you need **piers** to level that corner of the house. The longer you let that water sit there, the deeper the saturation goes, and the more expensive the foundation repair becomes.

Holy cow, seven years? I’m just starting to learn about home DIY stuff and that sounds like a total horror movie. I'm curious, did you ever notice a wierd smell in the bathroom or near that wall before you found this? I’d think seven years of soap and hair sitting in the dirt woud start to stink up the whole place eventually. Also, for the pros here—is it posible someone did this on purpose as some kind of ghetto "gray water" system, or is it always just a case of a lazy builder forgetting to finish the job? It’s wild that it even passed an inspection when the house was built.

Man, I’ve seen some lazy handyman specials in my time, but leaving a tub unhooked for seven years is a new level of negligence. To answer that last question, some old-timers thougt gray water was fine for the garden, but dumping it directly under the footer is just asking for a sinkhole. While you’re down there, check the vanity sink too. If the same crew did the whole bathroom, they might’ve cheated on the sink drain or the vent stack. Also, once it's dry, watch out for heave if you have expansive clay soil; the ground might move again as it dehydrates. Get a pro to look at the soil type.