Grass and soil grown inside septic tank

I have looked at a homestead property that has not been lived in for many years.

I looked at a what I believe is a septic tank - it's an above ground cylinder next to the house about twelve feet away and it was dry with grass and soil inside . There is also big tree next to it about 10 feet away.

What questions should I be asking?

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I don’t believe this to be a septic tank, as they are not normally above ground. If it is indeed a septic tank it should be connected to a French drain as a soak-away for the grey water, alternatively instead of a septic tank it could be a conservancy sewerage storage tank which is periodically pumped out.

I believe what you have is a storm water storage tank, which would have been connected to the rain water down pipe system off the main roof. The reason for the soil and grass inside could be a result of seeds having washed off the roof.

The only concern regarding the tree is the proximity to the house with underground root systems and dropping of leaves onto the roof and further blockage of gutters, etc.

Regards
Kathryn