Eureka, Spencer County, IndianaEureka is an unincorporated community in Luce Township, Spencer County, in the U.S. state of Indiana.HistoryEureka was laid out in 1858. A post office was established at Eureka in 1872, and remained in operation until it was discontinued in 1921. Tradition has it the community was so named after a pioneer exclaimed "Eureka!" when he came upon the site.GeographyEureka is located at.ClimateThe climate in this area is characterized by hot, humid summers and generally mild to cool winters. According to the Köppen Climate Classification system, Eureka has a humid subtropical climate, abbreviated "Cfa" on climate maps.