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downside
vs
underside

Downside is a noun that means the disadvantage of a situation.

For example: "One of the downsides of living in London, is that it is very expensive."

Underside is a noun that means the side of something that is usually nearest the ground.

For example: "Look at the underside of your iMac display. If you see an Ambient Light Sensor, you have a second generation iMac G5."