What I want:
My world has a large mountain valley region that is about 70 miles in diameter. I would like for there to be a historical event in which a portal to the moon was created in the valley and everyone living in the valley (and maybe some people outside too, doesn't matter) ended up dying of asphyxiation as air is sucked through the portal into the lower density atmosphere of the moon.
1) I would like the portal to be no bigger than a city block.
2) I would like the air depletion to be rapid enough such that few valley residents had time to escape.
3) If it helps, the elevation of the mountain valley town can be basically anything you desire, even below sea level.
Is the historical event I desire possible given the above parameters? If so, how big a portal are we talking? What time span?
[Personal note: thank you for reading! I've been lurking this site for a long time. Excited to finally get involved.]