The beautiful blue cobblestones lining Old San Juan are called "adoquines" in Spanish. These stones have served Old San Juan since 1784, when King Carlos III commissioned Royal Engineer Tomas O'Daley to examine the Old City's fortifications. O'Daley elected to dig up the city's old stone streets and repave them with the ballast from income galleons. The ballast was actually slag from iron furnaces in Spain, and were being used to keep Spanish galleons upright during the long voyage over. On return trips, the holds were filled with sugar and other items from the Caribbean. The adoquines acquired their bluish tint as a result of age and moisture.