Arcoiris is Spanish for rainbow.
As you may know, a rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc.
This fun geocache is at the North end of a big sweeping arc in the road (the top, on a N up map) ...a little upside down of the treasure at the end of a rainbow. :-)
Congrats to gw0143 for FTF within 40mins of published.