While Earth is the only place, as far as people know, that can sustain life; the planet has many regions where humans simply cannot survive. Oxygen, food and water are sometimes in very short supply due to harsh climate or toxic chemicals.
Unfavorable conditions are the reason many parts of the world remain unexplored or hidden from modern civilization. Such places are still a profound mystery. Even if they have a few inhabitants, they are not connected to the outside world.
Natural disasters and tragedies caused by man such as nuclear explosions and colossal fires have resulted in ghost towns that have been overtaken by Mother Nature but with no chance of residents returning back and resettling.
People are tough; humanity has survived inhospitable environments in the past. But even the strongest men won’t be able to survive for long in a desert where temperatures reach 145F every day, a place that is so cold it never rains, or in a city under which a fire has been burning and releasing poisonous smoke for decades.