Is abandoned sewage pit hazardous?
Do not enter the pit.
Without proper ventilation, old septic tanks (and this sounds not enough different to matter) are prone to collect gasses that will kill you (without you noticing. And the people who go in to try and help you, until one is smart enough to use a self-contained breathing apparatus to recover the bodies that died before them.)
It should have been filled with gravel to prevent access (and deaths) when it was decommissioned.
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