The popliteal space or popliteal fossa is located at the back of the knee. Pressure in this area often leads to fluid build-up, leading to Baker's Cyst. Usually, this condition arises as a result of other damages (injuries, surgeries, disease such as arthritis) to the knee area. The fluid surrounding the knee cap leaks and collects behind the knee, causing it to swell. The condition is not life-threatening, but may cause pain and discomfort.