The hole that stretched down from the vault to the ocean below had already been created during the battle.
When Kaneki had plunged his sword into the ground and created countless black tentacles, the layer of dirt below the vault had become severely weakened.
Thus, when Ren made his last stand while hanging from the ceiling, the mana generated from all the attacks headed toward him created a massive crater in the ground right below him.
Covering up the top portion of the hole was Ruby's lava, and right below Ruby's lava was Astrid's strong layer of ice.
The ice would be able to last for a few seconds before melting under the heat of Ruby's lava, but that was enough.
With the layer of ice preventing Ruby's lava from spreading down further into the hole created, Lily had finally raised her wand.
Showcasing her amazing accuracy when casting spells, Lily used her water magic to create a layer of water right on the ice floor that Alya had created.