How did Dumbledore know about the cave?

Wed Jul 27 08:10:14 UTC 2005

No: HPFGUIDX 135178

I was surprised when Dumbledore suddenly announced to Harry 
that they had to go to the cave, because I couldn't see where he 
found out all the info about it. It's tricky to get to, full of traps, 
rather unpleasant - but they just march off and get there straight 
away. I think I've missed something in the book, so maybe I can 
be pointed in the right direction?  I've had a couple of looks, but I 
can't find the right bit.

Sandra (confused at this hour of the morning already)