Science question about underground survival.

[Tonks]

OK.. let me try this again.

The President and the Congress has secret underground bunkers or 
something that they go into in the event of an attack on the country. 
In fact they used them during 9/11. Also submarines manage somehow to 
go under the sea and stay for a long time before coming back up. I am 
not a scientific person and I don't understand about oxygen, etc. But I 
know that we need it and there is a balance between the plants and us. 
On an smaller scale than the government has, how would a person live 
underground as far as oxygen is concerned if you had to concerned about 
poison in the air. And other than food and water, is that all they 
would need if they had to stay there for 6 months? 

Maybe I should ask the Slytherins since they spend a lot of time in the 
dungeons.

Tonks_op
I may be a bit crazy, but you have to admire my persistence.