Beneath a sprawling 14th-century building with moss-furrowed terrace walls and interiors painted in garish strokes of purple and yellow lie what could be the remains of Spain’s second-largest synagogue from the medieval era. The 7,500-square-foot estate, currently being explored by researchers and archaeologists, is nestled in a narrow alley in Utrera, a small city in the Seville province in southwest Spain. It sits covered with the geraniums popular in the city’s historic Jewish quarter. The property has had a lengthy and multifaceted history as a hospital, in the 17th century; a Catholic chapel; an orphanage, and most recently, in the...