Blocked Soil Vent Pipe ((top)) -

A soil vent pipe (SVP) — also known as a soil stack or drain-waste-vent (DWV) pipe — is a vertical pipe that carries wastewater and solid waste from toilets (and sometimes sinks/bathtubs) to the underground drainage system. Its top section extends above the roof, allowing sewer gases to escape and fresh air to enter, which maintains proper atmospheric pressure for smooth drainage.