There's a huge...crack...in my basement.

I've never seen exactly that issue before, but I have had occasional problems with uneven basement floors due to extending a basement without starting from a part of the existing basement. If the ground above isn't exactly level and you start a fresh basement up against an existing basement from uneven ground above the basement can become uneven. But I've never seen that long crack appear. It could be a shadow cast by lighting that lets you see where the floor changes level. So it might be a combination of a section of not quite level floor and the position of the lights to cause a shadow to be cast along the line where it changes level. Otherwise I have no idea. If my guess is correct then sims should be able to walk across it but you will get uneven floor warnings when trying to place an object on any square containing that line.