Human exploration of unknown lands and environment actually dates back to ancient times. But in a modern sense, when we speak about European early exploration, we rather speak about the sea voyages by the Portuguese, the Dutch, and the Spanish in order to find new routes to Asia, starting in the 15th century. Maritime trips were expensive and uncertain; the main goals of those expeditions were commercial.
Light appearing to reflect off a layer of water in the distance would constitute a "mirage," since this is a "fake" image that is not actually taking place, but instead is an illusion.
