You can see this in action with super-cooled water: the water is cold enough that it should freeze, but no collection of water molecules is able to start the ice crystal. To date, 15 different forms of water ice have been found, however here on Earth only one kind (“ice-I”) can form naturally. Even at the bottom of the Mariana Trench the pressure isn’t nearly high enough for any other kind of ice. The whole idea of Vonnegut’s ice-9 is that it’s a crystalline form of water, that grows rapidly at reasonable, Earth-like pressures and temperatures. Not surprisingly, Europa long ago underwent a colossal “ice-9 type catastrophe”, in that all of its water (at least on the surface) quietly froze solid.