Background keeps sinking

I’m doing a simple animation for class. It’s a person standing at a train station in a tunnel, and then a train passes by. Not complicated, but it’s what I need. Unfortunately, while I have managed to get the train to travel along the tunnel path without too much issue (just your basic selecting the wrong thing first for twenty minutes), I am having an issue with the tunnel. Every time I start animating the scene it sinks downward at a rapid pace.

I have no clue why the tunnel is sinking downward. Nothing else in the animation is having this issue. Please, can someone tell me why the tunnel is sinking and how to stop it?

Have you made it a rigid body ?
Check the Physics section in the properties window and remove anything you may have added in error

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