After a few minutes she drops back onto the shingle, bending her knees and lowering me down. She points to an opening in the rocks and I smile with realisation.

‘Cave, is that what you meant?’

She nods as if she said it right all along. ‘Ka-low.’

An echoey pit-pat and splash of water sounds from deep within and I get the impression it goes a long way back. It’s so dark and enclosed – why bring me here? I glance along the misty shoreline, wishing we could go somewhere else, when she walks inside.

I follow her and the ground shifts beneath me, my shoes sinking in deep silt.

Before me, a dozen or so boulders lead down like stepping stones. The air reeks of sulphur and rotting things. The further I go the harder it is to see, so that I can only just make out her broad back, as she picks her way over the rocks with ease, despite her bare feet.

Water drips on my head and trickles into my eyes. As I wipe my face, my foot slips on something squelchy and I grab the wall of the cave. I start to go after her, when I see something down below that makes me falter.

Glitonea is standing in a shadowy chamber, knee-deep in a pool of water, her eyes two points of eerie white light in the gloom. The darkness must be playing tricks on me as she looks completely different: her skin is flaky and peeling, like she’s covered in mud, and her hair is sparse and stringy. Water splashes and plops, tapping out a disjointed rhythm all around us. Why isn’t she saying anything?

The cave feels airless suddenly and I struggle to breathe. I don’t know what she plans to teach me, but I feel as if I shouldn’t be here. I glance over my shoulder at the entrance, wishing I’d stayed with Tom. I should have at least waited until he woke up. But then Morgan wouldn’t have told me to leave him and meet my sisters if he was in danger.

Glitonea holds out her right arm and a dot of golden light appears above her palm. It rises slowly, zigzagging one way and another like a firefly, before hovering high in the air. She mutters something and the light brightens, revealing a scene from another planet.

The cavernous roof is covered with stalactites. There are hundreds of them: fat white columns creeping down like melted wax, tiny needle-sharp shards packed together in rows, and huge icicle formations that hang over our heads like giant teeth. Dozens of stalagmites rise up from the ground, the pale mounds and columns filling the cave.