Gas under the diaphragm (pneumoperitoneum) is a red-flag emergency, most commonly due to perforation in a hollow abdominal viscus such as a duodenal ulcer perforation or intestinal perforation, allowing air to escape into the intraperitoneal cavity. Other possible causes include: Post-abdominal surgeries Infections with gas-forming bacteria Ruptured liver abscess Biliary-enteric fistula (rare) Pneumatosis coli (rare) Prompt recognition and surgical intervention are critical to prevent life-threatening complications.