Training and military simulations can immerse users in realistic visual worlds. Now researchers at the University of Tsukuba and the Advanced Telecommunications Research Institute in Japan have developed an interface that lets users negotiate such virtual spaces physically -- without bumping into walls. Four square tiles, about 70 centimeters on a side, zip around on motorized wheels to form a moving platform under a person's feet; the user can step in any direction but remain in the middle of the room. Ultrasound and optical sensors on the floor let the system predict where the user is about to step. The tiles communicate with a computer that synchronizes the person's movements with a virtual scene on a head-mounted display. The first application could be in virtual evacuation drills to prepare for disasters such as fires and earthquakes, says Tsukuba engineering professor Hiroo Iwata, whose team is working toward having a commercial product ready within a few years.