Everyone knows what water looks like. But simulating it digitally for the big screen can be tricky and time-consuming. At Digital Domain in Venice, CA, computer scientists Doug Roble and Nafees bin Zafar and their special-effects team have developed software that quickly creates large-scale, high-resolution water animations. Enter initial conditions of depth, volume, shape, and speed, and the software uses advanced mathematics to model a fluid's surfaces, realistically capturing phenomena such as complex currents and water swirling around buildings and people. An early version of the software was used to create the largest water simulation in a film to date: the flood in this summer's The Day after Tomorrow. The latest version takes five to 10 minutes to generate each frame -- one-third the time needed just a year ago, and with only one-quarter the memory. Roble says the software is now being used to do visual effects in two feature films due out in 2005.