The Cave Portal
The cave portal is the way Beatrice Tidal and Reagan Hideaway came out of Earth and into Eslura. It opens in a cave in the woods beyond Camp Tossbridge and lets out in a lake under two moons, and neither of them has ever understood it.[1][2]
The cave
Bea found the cave by running. She had fled the mess hall into the woods in a thunderstorm and spotted the mouth beyond a pair of birch trees. Inside, stalactites hang from the ceiling like crooked teeth, rotted vines and overgrowth gone pale grey from years in the dark drape the walls and snake across the ground, and at the back a tunnel drops away into blackness.[1]
The tunnel runs straight for a while, then twists left, right, up and down until the way back is gone, and finally spirals down into a large circular stone cavern of smoother rock.[1]
The crossing
Bea slammed her fist on the stone and cut her hand open. Everything went silent, an ambient golden glow filled the cave, and a figure surrounded by a golden aura stood in the mouth of the tunnel and called her name in her mother's voice, smelling of lemon drops and of home. She followed it down and it vanished in the cavern below.[1]
Then a five-fold symbol lit up on the cavern wall, five intertwining circles, pulsing dim and bright. Golden mist cascaded out of the stone where it was inscribed. Bea reached out and put her hand on it, the cavern began to shake and crack, and the wall took her arm to the elbow and would not let go. Reagan, who had come after her, caught her free arm and pulled, and both of them went through with a pop.[1]
What followed was a black void and a roaring wind, no light and no sound, then a flash of gold. They came down into icy water and surfaced in a lake in Eslura, a wood standing dense all around it and a large golden moon and a smaller purple one overhead. The five-fold symbol was on Bea's wrist when she came out.[1][3]
Afterwards
Bea calls it the hole they fell through, and wonders whether anyone at camp ever searched the cave and found it. She hopes they never did.[4]
Neither of them has been back. Reagan does not know where a way home would even begin, and admits she does not entirely mind the thought of staying.[5] Nobody in Eslura has heard of Camp Tossbridge or of the place Reagan comes from.[5]
Unexplained
The books do not say what the portal is, who made it, what opened it, or whether it can be crossed the other way. They do not say whether the golden figure was Celine Tidal or something wearing her shape, and they do not say where in Eslura the lake lies. Bea's own account of the crossing is a blinding flash of gold and her mother, and no more than that.[3] Reagan calls it a magic cave portal she cannot begin to comprehend herself and leaves it there.[5]