The BBC News article Isolated lakes found beneath Canadian ice sheet links to the open access Science Advances article Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic.
The discovery is based on radar "soundings" of the Devon Ice Cap in Nunavut, Canada.
An earlier question What is likely to have happened with these newly discovered antarctic volcanos below the ice? shows sub-surface radar measurements of antarctic ice, in this case to a depth of 1.5 kilometers, and therefore with a total path length of 3 kilometers in ice.
The water molecule is strongly polar and interacts with microwave radiation by absorbing and thermalizing it. This effect is obvious in microwave ovens. Microwave ovens can be used to "defrost" frozen food even a few centimeters thick, using reduced power and longer time. This is necessary because frozen water molecules have a lower absorption than those in unfrozen areas, but they are still quite absorbing.
Radar measurements travel through kilometers in ice round trip, substantially larger than the thickness of a slab of frozen peas.
How is ice-penetrating radar able to work through such long path lengths of ice? Is there a special frequency thats used, or other tricks allowing for extremely weak reflections after bulk attenuation to be detected and measured?