The Sea Witch's Cave is an underwater cave in Mermaid Bay, where Alex and Conner go to on the back of a giant turtle in The Enchantress Returns. They met with the Sea Witch to trade "Queen Red" for all her jewels, which they needed for the Wand of Wonderment.
"An enormous underwater canyon was ahead of them. [...] It had jagged edges and sharp rocks along its sides, and the canyon floor was littered with ghostly empty shells - it was like an underwater graveyard. [...] The cave was surrounded with small glowing lights, inviting and decorative. However, as they got closer, the twins realized the lights were the glowing antennae of a school of angler fish surrounding the cave. They were terrifying, with their sharp, toothy overbites and the spikes down their spines - they were the monsters of the sea."
The angler fish guard several species of trapped sea creatures. Inside the cave, there is a long tunnel guarded by white sharks. It leads to a space within the cave, filled with air, so the Witch's human customers can breathe without a mask. In this cave are dozens of mermaids, prisoners of the Sea Witch, hung upside down from the ceiling. Some are already dead. A large whale's rib cage acts as a staircase up to a smaller cave, with jellyfish carcasses draped over the entrance like a curtain. Inside is a rocky platform that acts as a small stage for the Sea Witch, who seems to enjoy looking down at her customers.