US scientists say they have identified a new human organ which they've called "interstitium".
Interstitium sits below the surface of the skin, looks like mesh and surrounds arteries and veins - a casing for fibrous tissue between muscles.
It can also be found lining our digestive tracts, lungs and urinary systems.
Until now scientists believed the interstitium layer was just a form of dense connective tissue, but a new approach to studying tissue allowed scientists to see it differently for the first time.
Historically, tissue samples were placed under a microscope, thinly sliced and treated with chemicals. But this approach would drain the fluid and collapse the structure of the interstitium.
A new method involving a camera probe has meant the organ can now be looked at as a living tissue.
Neil Theise, professor at New York's School of Medicine - the school that made the discovery - told National Geographic the organ is like a "shock absorber".
He has a theory it could be a fluid that supports immunity and that researching how diseases spread through this part of the body could give scientists a better understanding of cancer.
"Can we detect [disease] earlier by sampling fluid from the space? Can we figure out mechanisms to stop spread?", Professor Theise asked.