This is the site of Boston's first - and for a good while, only - flour mill. The Mill owners closed this pond off from the Charles River, leaving only a small stream that led to the ocean. Twice a day, the tides in Boston Harbour would fill the steam, powering the mill.
Of course, closing the pond off also made it stagnant. Since water was used as a dumping ground for sewage, garbage and dead animals, it was quite dirty and frankly, stank.
Around the turn of the century the city decided that something needed to be done. Rather than re-open the causeway and clean out the pond, developers decided they could make room for houses (and also more money) by filling it in with land taken from the top of Beacon Hill. So, that's exactly what they did. Building homes on sewage since the turn of the century! What a wonderful slogan that would have made.