How do I handle a mystery 4-inch waste line buried in my crawlspace?

This sounds exactly like an old farm tile or a legacy septic lateral that was never properly abandoned when the subdivision went up. Back in the early 2000s, developers were notorious for just grading over old infrastructure instead of digging it out, especally if it wasn't on the official plat maps. Since you're seeing sewage and standing water, you're likely dealing with **hydrostatic pressure** or a backflow from a line that is still tied into a larger network of old drainage tiles.

If the camera stopped 60 feet out at 6.5 feet deep, you need to know if it hit a hard cap, a crushed pipe, or a mass of roots. If it's a hard stop, that line is dead-ended, and the water is just surcharging back toward your crawlspace beacuse it's the lowest point of relief. Cutting and capping at the property line is the right move to protect your house, but you have to make sure you use a permanent **mechanical plug** or a solvent-welded cap. A cheap rubber Fernco cap might eventually blow off if the head pressure from the neighbor's side gets high enough during a heavy rain.

I've seen cases where these old lines were tied into perimeter drains for the original farmhouse. When it rains, the entire neighborhood’s groundwater might be migrating through that old vein of gravel or pipe right toward your foundation. You should also check if your neighbor has a sump pump or a footer drain that someone illegally tied into this 'handy' pipe they found while landscaping.

Before the excavator leaves, have them check the soil around the pipe. If the dirt is saturated or 'sour' smelling, that pipe has been leaking into your soil for years. Once you cap it, keep an eye on your crawlspace for a few weeks. If the water stops but the smell stays, you might need to treat the soil with some lime to neutralize the odors trapped in the dirt. But definitely get that cap on there—you don't want to be the neighborhood's unofficial overflow basin.

I’m still trying to wrap my head around how all this works since I’m a total newbie. If you cap that pipe at the property line, does the water just sit there and build up forever? I’d be terrified it whould eventually burst somewhere else or just turn the neighbor's yard into a swamp. Also, you mentioned a mechanical plug—is that somethig a regular person can install, or is it super technical? I’m still learning the basics, but the thougth of pressure blowing a cap off sounds like a disaster waiting to happen. How do you even check if the pressure is getting too high once it's buried?

While the mechanical plug suggestion is sound for temporary isolation, you must strictly ensure that any solvent-welded cap utilized is preceded by a thorough cleaning with an appropriate purple primer to guarentee a chemical bond capable of withstanding the hydraulic head that will inevitably accumulate once the downstream flow is restricted. I woud be particularly concerned about the specific gravity and effluent composition of the 'sewage' you mentioned, as it might indicate a cross-connection where a neighbor's ejector pump is inadvertently discharging into this legacy lateral. Before you backfill, I'd strongly advise installing a check valve or an accessible cleanout riser at the property line so you can monitor for any persistent surcharging without needing to excavate your crawlspace again.

If you're worried about that cap failing under pressure, don't mess around with generic stuff. I always use an Oatey 4-inch PVC DWV Cap for these ghost lines. You need to use thier Purple Primer and Medium Orange CPVC/PVC Solvent Cement to ensure that bond is permanent. If you just shove a rubber cap on there, the groundwater pressure from your neighbor's yard is going to pop it like a cork during the next big storm. I’ve seen it happen. Grab the Oatey stuff and you won't have to dig that spot up again in five years.

Whoa, a ghost pipe! That is so wild and honestly kind of spooky. I just joined beacuse I'm trying to learn the ropes of DIY plumbing, and this thread is better than a mystery novel! Since you're digging anyway, has anyone ever found anything cool like old coins or artifacts while excavating these old farm lines? I'm curious if those old pipes ever contain anything other than just... gross stuff.