The Cracks appeared all at once, all over the world, and, as was later discovered, some distance above and below it as well.
They couldn’t be closed or widened, (although eventually crude gates were made) and they didn’t only appear in solid objects. No one knew what created them and no one ever found out. More importantly, going through them didn’t just lead to the obvious other side on earth.
A few people went through by themselves without preparation, sometimes by accident, and some of these got lost or met an accident. Some survived and returned however, some of them had only intended a quick look anyway. Most governments however sent scouts through, and tried to wall off public access to what Cracks they could. Some governments just did the walls without the scouts.
Most visual descriptions of the other side used words like “flat”, “empty”, and “colourless”, and “black”… And also “weird” and “strange”. Fortunately there seemed to be breathable air, although no sign of what made it breathable.