There is a blockage.
Your drain waste vent is plugged or leaking somewhere. The drain waste vent is a pipe that carries sewage gas out of your home, exiting usually the roof but sometimes the side of the house. Get it fixed asap, this is not good for you.
There will be the connection to the sewer and the drain coming out of the house. Connect the two with PVC at the proper pitch. 1/8 inch fall per foot of length.
The drain from the toilet to the main drain is plugged and the branch to the shower is not. If the main floor toilet flushes, it is between the two toilets and not between the house and the sewer.
Rain
Find the condensate drain and make sure it is open. If there is no condensate drain, figure out how to drain the condensate.
The trap is a P or S shaped bend in the drain pipe which retains water to block gases from coming out of the drain. Traps are located under the sink, under a shower or bath tub, and they are formed into the drain path of the toilet.
it may mean that the sewage coming from your toilet is over flowing and you'd need to call a plumber immediately! Or you have gophers.
drain or gully may have another inlet coming into same pipe via a junction under the floor
I'm looking for the same answer. The clay 'French pipe' along the perimeter of the crawl space seems to be full of and leaking sewage (black gunkish water). How can a pipe laid to drain ground water even contain sewage? cfgeog@wowway.com
Sewage from all points of source in a house will be collected in a single ground level collection pit within the house premises which will be connected to the public sewer by pipes laid underground to convey the sewage. A silt trap with a silt bucket will also be installed in the system to remove the silt carried by sewage and accumulated in the pit before the sewage flow into the house service connection pipes to prevent clogging of pipes.
A condensate drain pipe is used in conjunction with air conditioning equipment. Usually installed with heat pumps, this drain pipe gives condensation a place to run off.
== == == == == == If you have an open septic tank or broken sewage pipe anything that crawls can get into your toilet or bath tub or sinks. You can buy a special trap device to keep all creepy crawlers out of your sewage pipes. The device lets the water out but it won't let the critters in. Keep a tub screen on the drain in your tub to reduce spiders from coming into your bath tub.I have seen a snake come out of my own toilet. Spiders come up out of the bath tub drain all of the time. I live in the woods. You can expect the unexpected when you live in a forest.