Ultrasound Beyond the Speed of Light

Interesting!

"The speeds of sound in most plastics are also some 5 orders of magnitude shy of c. So it may be hard to believe that by sprinkling in some small plastic beads, water can be made to support ultrasonic pulses with speeds faster than light. Not only can ultrasound pulses outrun light in this watery mixture, they can also propagate in negative time, apparently reaching a more distant point before a closer one. How is this possible? What does it mean? How does it fit within the known laws of physics? In the remainder of this article we will attempt to answer these questions."