It's not hard evidence yet, but the scientists believe a lake fits the data best. Further investigation will be required to verify its existence.
There are other possible explanations for this sort of signal, but the authors consider and reject them. To give one example, they consider a layer of carbon dioxide that was liquified by the pressure. While this layer could form under the conditions present at the base of the ice cap, its properties are such that you wouldn't get such a strong reflection. Other conditions they consider would produce similar reflections but are unlikely to be found under a 1.5km thick sheet of ice.Via: ARS Technica