A sink hole swallows a Quebec Family. Much of Quebec is above a clay formed by an old sea that makes it vulnerable to collapse.

from NYT

Because the clay formed in salt water, Professor Bouchard said, the molecular structure of its particles resembles playing cards arranged as an unstable house of cards, rather than stacked in a deck, as occurs with clay formed in fresh water. A variety of events can break the molecular bonds holding the clay particles together. When that occurs, the clay can spontaneously liquefy with little or no provocation.

“Even a fly landing on the surface can set it off,” he said.