Deep-vein thrombosis
a noun
Pathology .
1. A condition in which a blood clot forms in a vein deep beneath the skin, typically in the leg or pelvic area: Immobility and lack of exercise are risk factors for deep-vein thrombosis.
Usually "clot busters" are used. And they do go away. They can travel to the lungs or other parts of the body causing worse conditions.