(CNN) It's raining plastic -- that's what a survey of rainfall in Denver and Boulder, Colorado, concluded recently.

The rainfall survey, titled "It is raining plastic," was put together by scientists at the US Department of the Interior and US Geological Survey.

They couldn't see the plastic with their naked eyes, but found it using a binocular microscope fitted with a digital camera.

They found plastic showed up in 90% of the samples, mostly in fiber form, and came in a variety of colors. Blue was most prevalent, followed by red, silver, purple, green, yellow and other colors, the study says.

It's unclear where the plastic is coming from, but plastic contamination has been a growing problem throughout the world.