There’s no telling what happened when/if they broke through. The kind of pressures that were trapped in there would be difficult to deal with and reducing the pressure in there would bring other consequences.
Russian "scientists" on the move.
READ THIS:
One major concern is the Russians have filled the hole they’re drilling with more than 14,000 gallons of kerosene and Freon to prevent it from freezing shut. The Russians have engineered their system so that when they break through into the lake, water pressure from below is supposed to push the drilling fluids up the hole, rather than letting them pour into the lake and contaminate it.
SOURCE: http://www.npr.org/2011/02/03/133441327/deep-below-antarctic-ice-lake-may-soon-see-light
(and that is from 2/3/2011 A YEAR AGO. They were 100 feet away from the lake)