** It has septic and drain fields and sub pumps the whole 9 yards. I have this tank or hole in my back yard that we don’t know EXACTLY what it is. **
The tank holds solid waste (poop, toilet paper, etc.) from the house drain system. The hole is so you (or someone you hire) can clean it out when needed (or periodically, to avoid blockages).
You might want to do an Internet search on “septic system” to read up on how they operate and how they need to be maintained.
JMO
Dan O.