News flash: Despite what we’ve thought our whole lives, skin may not be the body’s largest organ. Some scientists now think that designation belongs to the interstitium, a mysterious and complicated layer of our body that appears to flank all other organs—including the skin.

If that’s indeed true, the interstitium is our largest organ by volume. Which begs a couple of questions: What is the interstitium? And how have scientists missed it till now?

In the journal Scientific Reports, researchers describe the interstitium as “a previously unrecognized, though widespread, macroscopic, fluid-filled space within and between tissues.” It’s made up of collagen bundles surrounding fluid pockets that may contain up to 20 percent of all the fluid in our bodies. Before the researchers discovered it by examining immediately frozen biopsy tissue, it was assumed to be dense connective tissue—not the unique, malleable network that it is.

Much more research is needed to determine the interstitium’s purpose, but scientists suspect it may serve as a shock absorber for other organs. They also think it may be a mechanism for cancer to spread beyond the organ where it originates—which makes the study of this “new” organ seem imperative indeed.