We moved a few months ago to a property that has bore water, and in the backyard is this, which we were told is to do with the bore - but what it is it for exactly ? And if I put decking over it, will it cause any issues ?

Its full of mud/sludge atm. Theres a entry/exit pipe on either side of it. 

Some sort of overflow for the bore ? Apparently its an extremely deep bore compared to the rest of the community.