The new paintings weren't found in a secret room or beneath debris. In fact, they were right in front of everyones' faces this whole time, but faded enough that they couldn't be easily seen with the naked eye. Instead, scientists used a photo-enhancement technique to make the difference between colors more apparent, bringing long-lost paintings into view.
The big reveal was made by art expert Noel Hidalgo Tan, who was working at the site and spotted odd traces of reddish paint on the wall on day during his lunch break, LiveScience reports. Tan took some photos and didn't think much else of it. When he later uploaded the images to his computer and played around with them using a color-enhancing technique called decorrelation stretch analysis, he was shocked to find an entire menagerie of detailed imagery.
It's not the first time the technique, called decorrelation stretch, has been used in science. NASA uses DStretch to enhance photos from Mars and archaeologists the world over use it to make faded, ancient petroglyphs easier to study. The picture above this paragraph is a petroglyph image from Baja California enhanced with DStretch. Below, you can see what that same image looked like without the technique applied. It's easy to see how paintings could go totally unnoticed without it.