There is a way to explore the world over the alpha map…
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In the game console (~) you can change gravity value with command of p_gravity_z XX, where the XX is gravity value. Normal value is -13. For now:
Set gravity to value of 10, so you can fly up… Fly to the altitude over
100 metres and then set gravity to 0, so you stay in the air in same
altitude and you can fly forward,backward,left,right, so you can step
over the border, set normal gravity (-13) and explore world outside the alpha map… Enjoy 
And here is video manual
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