Every drain in your home has a "P-trap"—the curved pipe underneath the sink. This trap holds a small amount of water that acts as a seal against sewer gases. If a clog in the vent creates a vacuum (negative pressure), it can suck the water out of the trap (known as siphoning), allowing methane gas to seep into your home. If your bathroom smells like rotten eggs, check your vents.
If one drain is clogged, it’s usually a local blockage. If the toilet, shower, and sink are all backing up at the same time, the issue is likely in your main vent stack or sewer line. clogged drain vent