There are cosmic events so powerful that they leave permanent marks on the fabric of the universe itself. Imagine two colossal black holes spiraling into each other, yes they send ripples in the fabric of spacetime—gravitational waves that we’ve only recently learned to sense. Ripples pass, leaving the pond … or the universe … unchanged when they’re gone. But ripples aren’t the only type of wave. There’s another type of wave that leaves a permanent mark—a memory etched in the fabric of the universe. They’re akin to gravitational tsunamis, and we’re on the verge of being able to detect them.