Light Wells are huge deposits of Light into the world. They are found in the deepest corners of Mundis (or in plain site; it doesn't really matter) and are typically rather complex caves made entirely from Light. Entering one unprotected would be fatal. Wells (such as these and Chaos Wells) disrupt the balance of Chaos and Light in the world, and are ultimately unfavorable in most eyes.
Light Wells are the only reliable places in the world where one can find a Night.