In fact, these are actually taken using x-ray technology. I don't completely understand it, but this is what Steven Meyers says on his website:
By using x-rays instead of light, and unusual innervision can be revealed, and nature shows us textures, details, and shadows that would otherwise not be seen. Visible light is just a small part the electromagnetic spectrum, and falls between x-rays and infrared. My x-ray images are a collection of negatives, positives, and solarized images, solarized being partly negative and partly positive at the same time.