Hollyleaf was upset that she was half-Clan because picking a mate from another Clan is against the warrior code, and Hollyleaf feels a strong connection with the warrior code, so she headed to the tunnels to escape her clanmates. Lionblaze and Jayfeather followed her but before they could stop her she went into the tunnels and the tunnels collasped on top of her. Lionblaze and Jayfeather told the rest of Thunderclan that she chased a squirrel into the tunnels and it collasped on her.
There are rumors, though, that Hollyleaf is still alive and she alive
No, it has not. It is unknown what her fate was in those tunnels.
There are rumors that even though Jayfeather saw her get crushed by the stones, she is still alive. Actually, Jayfeather didn't see her die. He just heard a rock-fall. Did the Erins plan it this way?
Erin confirmed that hollyleaf was one of the main characters so its most likely that she's still alive
She is NOT DEAD. Dont beileive then read Sign of the moon. There is highly strong evidence she's alive. - Solthemortal
The site is not always taken as canon. Remember that the family tree on that site is incorrect as it still lists the three as the kits of Squirrelflight and Brambleclaw. Her status remains unknown until then. We will have to wait and see. - Aleu
She ran into the tunnels where Lionblaze and Heathertail played. It is unknown whether she survived or not.
In Sign of the Moon Jayfeather and Lionblaze go into the tunnels to try and see if Hollyleaf's body is there or not. They don't find her body but manage to find a tuft of her fur. They assume that she's alive but it's not yet clear. Later when Blossomfall and Ivypool go into the tunnels an unidentified cat leads them out. It's unknown if that was Hollyleaf or not. --BlizzardpeltIcetribe Fighter 20:42, January 8, 2011 (UTC) 23:23, April 10, 2011 (UTC)
HOLLYLEAF DID NOT DIE IN THE TUNNELS! She survived the the rock fall and lived in the tunnels with Fallen Leaves for a long time. She helped Blossomfall and Ivypool and then Dovewing and Ivypool out of the tunnels. She comes back to ThunderClan in The Forgotten Warrior alive and well. She later dies in the Battle of the Dark Forest.
By Blacktail, Medicine Cat of MountainClan
All my information came from Warriors Wiki