found something weird under basement floor

hi, i am new to the forums

i bought a house in michigan about 2 years ago. it has a partial basement, the house was made in 1945.
in the basement there are 2 approx. 2ftx1ft rectangular cutouts in the basement floor, they are covered with cement covers that have 2 screws in them. the covers are flush with the floor.
i got curious today and pulled one of the covers off to see what was under them, i expected to see pipes or just dirt, anything but what i saw.

what i saw was a large and deep hole under the floor, there were pillars or pylons that support the basement floor. the hole was filled with water, i wanted to see how deep it went so i stuck a broom handle into the water and it went all the way down, when i pulled the handle out the handle was covered in some type of black/blue sludge that reeked almost like fuel oil or grease, and had the consistency of watery mud. the smell made me feel slightly dizzy. i put the cover back on and left the basement and ive been trying to figure it out.

what do you people think this is??
this house was built in 1945 and at one time used a septic but is now on city sewer, or at least i pay a sewer bill. the house is still on a private well for water.
could this be the old septic?? ive never heard of a septic under the basement floor though. there is a floor drain in the basement and it looks like the sewer pipes from my kitchen go under the basement floor also.
any light that anyone can shed on this would be great.
thanks, josh

No clue, that sounds creepy though.

Welcome Josh. I used to live in Waterford. Around Hatchery & Dixie Hwy. Unfortunately, I don't know what the holes are for. I might guess that they were at one time connection points for a fuel oil tank?

Ok, sounds interesting enough.
Do you have an oil fired boiler or water heater? And do those lines from the tank go into the basement floor?
Sounds to me like an oil separator under your slab....but why is a good question.

Sounds like possibly an old septic tank although under the basement is strange place for a septic. I think I would call a septic pumper and have them pump it out. Then fill it with sand or dirt.

Gross! I would've been afraid to 1st open it up, then 2nd -stick something in it. Yuck! Keep us informed if you find anything out. Maybe ask some neighbors with similar age houses if they have anything like that.

i called up a plumber/septic installer, he said it may have been a grease trap at one time. he said that he doesent know what it is, but either way if its not causing any problems then to leave it alone and DO NOT call anyone over to investigate, as they would be under obligation to report anything that isn't up to code to the city, which means the city would make me upgrade it.
i called my father in law over, he is kinda like the macgyver of home repair, and generally a "hack of all trades", he said that the sludge is sewer sludge but he doesent know where its coming from or why it is down there.
The pipe that runs into the basement floor is only about 1.5 inches, so i don't think its a waste line from the house, and it appears to exit through the foundation wall underground. he said he thinks it looks like some sort of vent pipe but he isnt sure.
About 2 feet from the pit is a floor drain, i stuck a pipe down in there and it went down about a foot or so and when i brought it back up the end was covered in sand.
upon closer examination the pit is hexagonal in shape, about 3 feet around maybe, and about halfway up (flush with the water line) is a large diameter pipe running horizontally towards the other opening that is about halfway across my basement. i assume that under that opening i would find the same thing.

possibly related, a couple years ago when i was digging a new garden bed in my back yard i unearthed a large pipe, maybe 6 inches in diameter, running vertically through the ground with a loose fitting cap on the top of it. the end of this pipe was about a foot under the surface of the soil. i thought this maybe was the old septic but im not sure.
the septic guy i talked to said that typically they would have to either backfill or remove the old septic system.

i am content for now to leave it alone...i just wish i new what it was in the first place.

thanks
josh

Yuck ! Weird!

Some houses were built without perimiture drains. Has this house been raised? If it has these could be just a collection point for water under the floor. It sounds like they were placed before the floor was laid. This is often done when adding a basement under a house, instead of digging the outside. You should be able to have the sludge tested to deturmin if it is dangerous.

I would also recommend spending a few dollars and get it tested. If found to be harmful, it would be in your best interest to get it pumped out and keep your family healthy.

Close it back up and forget about it...
Don't mention it to anyone and sell the house as fast as you can....

Your house is on an old toxic waste dump....

You aren't in the area between Watkins Lake and the Airport are you?
Thats where they dumped the really bad stuff...

I'm just joking here but If I was you I sure would want to know what this is...

Do you have any pics? I think it would really help here.

Didnt this come up when you bought the home? I know you said you just found it so maybe you did not see it when looking at the house but I am surprised the previous owners did not disclose this...did you sign an arbitration form? You could take them to court. I am surprised your inspector did not mention this either. I know it is somthing you are leaving well alone but I am worried this may eventually make you sick or if you go to sell you will have to disclose it and fix it anyways.....arbitration may get them to pay for it?