Amarillo's name probably derives from the nearby Amarillo Lake and Amarillo Creek, named in turn for the yellow soil along their banks and shores (Amarillo is the Spanish word for yellow, the color) or the yellow wildflowers that were plentiful during the spring and summer. The city once self-proclaimed as the "Helium Capital of the World" for having one of the United States' most productive helium fields. The city is also known as "The Yellow Rose of Texas" and most recently "Rotor City, USA" for its V-22 Osprey hybrid aircraft assembly plant.