Leak below floor underneath kitchen unit?

We have a hump in our Amtico floor,a very damp floor under our sink with the damp radiating in a semi circle upwards from the floor on the external wall. There are pipes for an outside tap, dishwasher[aquastop type] and the sink piping with stopcock all which are above the damp level and are dry. What is wrong and how can we fix it? We have a granite worktop which could not be moved.