What does Mark tell Helen his job is?

Mark tells Helen his job is in the hole.

He pulled her around to face him, so that her upturned mouth was inches from his own. "That's just it. You can't stay with me. If and when it comes I'll be in the Hole, protected by fifty feet of concrete and steel and good earth. That's where my place is and that's where you can't be. You'd be somewhere on the surface exposed. If you could come down into the Hole with me, then you could stay, darling."

This was something he had not said before, a fact she had not considered. Somehow it made her feel a bit better, yet she continued to argue, although dispiritedly. "Still, I think my job is here."

His fingers banded her arm and when he spoke his voice was flat, a direct order. "Your job is to survive because if you don't the children won't survive. That is your job. There is no other. You understand that, Helen?"