If you demolish a background tile (tempshift plate), it doesn't matter the location on the map, you will open a hole to outer space. I built then removed tempshift plates in my crude oil reservoir, I looked away, then when I came back, the oil was gone and there was only vacuum. I did a second test on a water reservoir and got the same result. In both cases, I was way bellow the "space" line, deep underground, close to the lava. The background graphics remain the same. You don't actually know that you've hit outer space until you lost a bunch of resources.
Build then demolish a tempshift plate. You might have to use the demolish tool and set it to "background buildings".