A belly in a sewer line occurs when a section of the pipe sags or dips, creating a low spot where wastewater can accumulate and cause blockages. To fix a belly in a sewer line, the affected section of the pipe needs to be excavated and replaced. A professional plumber will typically use a specialized camera to locate the belly and determine the best approach for repairs. It's important to address a belly in a sewer line promptly to prevent further damage and potential sewage backups.