Answer: Continuous diaphragm sign (pneumomediastinum)

You should NOT see the medial diaphragm normally because the heart sits on it. If you DO see it, it’s ABnormal. If the lucency is above the diaphragm, think pneumomediastinum. If below, pneumoperitoneum can be similar.

ref: https://radiopaedia.org/articles/continuous-diaphragm-sign?lang=us