Does this basement pit drain into the soil or the sewer?
I emptied the water from this pit and after a day or so it started to smell like sewage in my basement.  That's when I realized that this pit works the same way as a toilet.  The water at the bottom block the fumes from the sewer.  The oil-like substance at the bottom of the pit is some kind of sealant put there to prevent the water from draining into the ground below.  I dumped some water down the pit and voila, problem solved.