You’re at home and you catch an unpleasant whiff of… something. Most smells come and go, but if bad odors persist and become unbearable, it’s time to figure out what they are. How can you identify something you can’t even see? The following is a selection of descriptions of strange smells around your home and what they might indicate to help you figure it out.

Eggs Gone Bad

The first smell to be concerned about requires the swiftest response. If you smell rotten eggs, you may have a gas leak. On its own, natural gas smells like nothing, so a chemical called mercaptan is added to give it that horrendous bad egg smell as a warning. Gas leaks can happen almost anywhere in and around the house. When you smell it outside, look for bubbling water or dead plants near where the gas line rests. Long-term exposure to natural gas is extremely unhealthy, but the bigger problem is the risk of explosions. So call the gas company and get out of the area until an inspector arrives.

Rotten Eggs… and Worse

A working septic tank will never smell, and you should be able to live your whole life in your home without worrying about it. But when it’s not working properly, you’ll know it. Septic tanks hold all the wastewater that leaves your home—that means everything that drains through the sinks, baths and showers, washing machines, dishwashers, and, yes, the toilets. The tank holds the waste and allows the water, grease, and sludge to separate before draining it to the main sewer line. If you smell something foul in your bathroom or kitchen, it might be methane, hydrogen sulfide, or other gases backing up from the tank. Call a septic tank specialist for an assessment and possible cleaning.

Smells Like Death

Unfortunately, death might be exactly what this smell is. If you detect a sickly to overwhelmingly rank smell coming from the walls, attic, or vents, a critter may have crawled in, gotten stuck, and died. As it decays, it emits a foul odor that gets worse in hot weather. Before this happens, check your home for uncovered entry points—such as spaces around pipes, windows and doors, chimneys and vent pipes, and other spots—and eliminate them with caulk, panels, insulation foam, or other material. If a creature does get in, call an exterminator or, if applicable, an HVAC expert. They can recover the body and prevent the situation from happening again.

Musty and Moldy

When you’re thinking about strange smells around your home and what they might indicate, keep your nose alert to mold. If you sense a wet and dusty scent in the basement, in the attic, or elsewhere, start looking for mildew and mold buildup. Both mildew and mold can occur wherever moisture is allowed to build up, whether through leaks, humidity, broken pipes, or leaky appliances. Set up fans and a dehumidifier in these spots. You can clean up small amounts of mold or mildew with disinfectant, but an expert should look at large outbreaks.