They also found that narrow pathways through the ice shell can remain open from the sea all the way to the surface, if filled with liquid water.
These open pathways are perfect channels to explore the hidden ocean below.
why not just burn the ammonia?
Actually this is possible. From wikipedia:
Ammonia cannot be easily or efficiently used in existing Otto cycle engines because of its very low octane rating, although with only minor modifications to carburetors/injectors and a drastic reduction in compression ratio, which would require new pistons, a gasoline engine could be made to work exclusively with ammonia, at a low fraction of its power output before conversion and much higher fuel consumption