On Tuesday, The New England Journal of Medicine tweeted the most recent addition to its photo series of the most visually arresting medical anomalies. The image is of a mysterious, branchlike structure that, posted elsewhere, would probably pass for a cherry-red chunk of some underground root system or a piece of bright reef coral. But this is no creature of the deep. It’s a completely intact, six-inch-wide clot of human blood in the exact shape of the right bronchial tree, one of the two key tubular networks that ferry air to and from the lungs. And it was coughed up in one piece.

The clot is beautiful, and it’s also kind of gross. The tweet received a slew of replies from those frightened that the photo showed an actual coughed-up lung, which is about as likely to happen as your brain falling out of your butt. But even the doctors who treated the 36-year-old man who produced the clot aren’t entirely sure how it could have emerged without breaking.