Virtual reality can be used by teachers to easily present complex data to students. Healthcare is a major user of VR for surgery simulation, phobia treatment, and skills training. The history of VR began with devices like Sensorama and early head mounted displays in the 1960s, with continued development of technologies like head mounted displays, CAVE environments, and virtual reality modeling language (VRML) for the web. VR can be immersive, involving complete immersion in virtual worlds, or non-immersive by adding digital layers to the real world.