Underground water in my garden?

Hi, we have just had our garden landscaped and a small area of ground which is surrounded by slabs to put plants in is very boggy, so much so we can sink a foot into the soil. We dug down and have found 2 small areas close to each other that fill with water which is oblivously why the soil is so boggy. There are no water pipes in the immediate area and the house was built 30 years ago on farmland. Anyone have any ideas where the water is coming from amd how to rectify the problem. Is it a natural source of water or a serious problem. Only constructive suggestions only please, thanks