Now I remember why I didn't finish this game on my first play through and I made the mistake again! :(

You can go back down there.. I just did that last night when I missed the cache the first time. The only problem is that you have to go back out the "entrance" and then enter the "exit" to trigger the next bullet point in the quest.

Ah, darn I ended up re-starting. I am at Chernobyl. I need to wrap it up one of these days.