In the National Geographic article "Newly Discovered Organ May Be Lurking Under Your Skin," Catherine Zuckerman details the discovery of the interstitium, a vast network of fluid-filled cavities found throughout the human body.

Long dismissed by scientists as simple connective tissue, researchers from New York University’s School of Medicine suggest it should be classified as a distinct organ. The interstitium is located just beneath the skin's surface and also lines the digestive tract, lungs, and urinary systems. It consists of interconnected compartments supported by a meshwork of strong collagen and flexible elastin proteins.  

The discovery remained hidden for decades because of traditional microscopic techniques. When tissue is sampled and put on slides, the fluid drains and the structure collapses like a flattened sponge, making it appear solid. Researchers only identified its true nature by using a new "virtual biopsy" technique called confocal laser endomicroscopy, which allows for the observation of living tissue in real-time.  

Functionally, the interstitium acts as a "shock absorber" to protect organs from rupture. Crucially, because it serves as a fluid highway, scientists believe it may help explain how cancer spreads through the body, potentially leading to new diagnostic tests and treatments for various diseases.