Complex sentence: San Andrés was discovered sometime before 1527 by Spanish voyagers but it wasn't until this year that the islands began appearing on Spanish maps; even Dutch are reported to have come to these islands at the end of the 16th century.
Split into simpler sentences: San Andrés was discovered sometime before 1527 by Spanish voyagers but it wasn't until this year that the islands began appearing on Spanish maps. Even the Dutch are reported to have come to these islands at the end of the 16th century.