User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5402289-20121122023001/@comment-3146930-20121122041749

Except Nico survived under different circumstances. He was pulled into the pit because he didn't fully understand its' power, was captured by Gaea's forces, and then brought to the Doors of Death in Tartarus before being brought back to the mortal world and used as bait. He didn't survive because he was smart or strong, he survived because Gaea had use for him.

Percy and Annabeth may survive in Tartarus simply because Gaea may want to use them and capture them in a similar way, except she could capture them and then bring them to Olympus so she can kill them and then rise from her sleep.