OTTAWA — Richard Préfontaine and his wife, Lynne Charbonneau, were watching a playoff hockey game with their two daughters on Monday night when the ground beneath their house gave way suddenly and without warning.

The house’s bright green metal roof was all that was visible the next day in a vast mud crater near the village of St. Jude, Quebec, about 50 miles northeast of Montreal. The landslide created a hole 100 feet deep, 300 yards wide and a third of a mile long.

The family’s remains were found huddled together on a couch by the television, with rescuers discovering only their golden retriever, tied to a tree, alive.

On Wednesday, officials allowed residents of several nearby houses to return home. But the family’s shocking demise was a stark reminder of a hidden menace under many parts of Quebec, one that dates back 10,000 years to an ancient inland sea.