Robert Gray wrote:

Interesting idea - 'big hole filled with rocks and sump pump'

My first reaction is to say no, definitely not a failed septic line. It is
more than 100' from our septic tank, and on the other side of the house.
But, who knows?

100' from the tank is nothing when you consider the lines running from
the tank are several hundred feet long. Is this spot up or downhill
from the tank? If downhill, is there a big drop, or a gradual one? Any
chance you could find the original septic permit, from builder or
possibly county, to find where the drain field was supposed to be
located?

Chuck