Hernando de Soto ( Jerez de los Caballeros , Badajoz , Spain , c. 1496\/1497 â `` May 21 , 1542 ) was a Spanish explorer and conquistador . He lead the first European expedition deep into the territory of the modern-day United States and was probably the first European to discover the Mississippi River . Hernando de Soto ( c. 1496\/1497 â `` 1542 ) was a Spanish explorer and conquistador who , while leading the first European expedition deep into the territory of the modern-day United States , was the first European documented to have crossed the Mississippi River .