What kind of gas can you smell but cant see?

One common gas that you can smell but can't see is carbon monoxide (CO). It is colorless and odorless, making it particularly dangerous as it can accumulate in enclosed spaces without detection. However, some gases like natural gas, which is mostly methane, are odorless as well but are often treated with a sulfur compound like mercaptan to give them a distinctive smell for safety. Always ensure proper ventilation and use detectors for safety when dealing with potentially harmful gases.