Outside, the sun frazzles the beach and the promenade. The tide is way out, and everyone else seems to be taking advantage of the expanse of sand. They don't seem to care about what's in the ocean, only about lying face-down next to it. You've never understood that idea.
Here it is dark and cool. You walk on stone slabs, and the chill rises until your sandalled feet are just this side of uncomfortable. You hang back, letting the rest of your group get well ahead of you. An hour in this wonderland is a delight that sharing will spoil. You wait until the sound of voices has faded, and you let yourself begin to look.
This is a treasure house of living jewel boxes, kaleidoscope worlds of shifting colours and darting shapes: all manner of fish, vivid yellow or bright orange, or streaked with scarlet and electric blue.
There's a tank that looks empty. It isn't. At the very bottom there is a fragment of earthenware pipe, half-buried in the sand, furred green with weed. You hold your breath and watch. A solemn blue lobster extends a claw and a whisker, then appears to think better of it and retreats again.
Next to it: a smaller tank, where seahorses curl their tails around thin, grassy weeds. A simulated rockpool, bright with the delicate fronds of sea anemones.
The next room is different. Bigger. The water is held back from you on either side by great glass walls. The fish in here are larger, more timid. The long shimmy of an eel. Further back there are creatures whose shapes you can't quite make out, lurking behind rocks.
It's dark in there, lit only by discreet lamps in the corners and the occasional phosphorescent glow of a jellyfish drifting past. Bubbles rise, gleaming, out of sight. The glass extends up above you and curves over your head: you are standing in the centre of a tunnel made of the sea. As you look up, a dark shape moves purposefully across the liquid ceiling – and it's gone, a neat flick of the tail sending it out of your sight – and you cross to the other side of the corridor to see where it went.
It is so dark in there that you cannot see the back wall. There could be more rocks there, or weed, or anything, or nothing. The green water becomes opaque. A silvery grey fish swims out of the murk towards you, gleaming briefly as it turns back. You do not recognise it; you follow it with your eyes, trying to work out what it might be.
Your own reflection is irritating. It gets in the way. You approach the glass as close as you can; you press your nose up against it, but this ghost still floats in front of you, blocking your view of the fascinating world inside.
The sign says: do not tap on the glass.
Imagine. Imagine if you did. Imagine if the glass were not glass at all. Imagine if you tapped on it and you found your hand passing through, and your arm, and – before you could scream – your whole body was drawn through into the water, and the wall closing behind you.
You would wonder why you weren't drowning; you would decide not to think too hard about that, in case you suddenly found it happening. You would look back through the glass that wasn't glass. Did anyone see you? You were alone at the time. It's difficult to see where the walkway is; from here it just looks as if the water goes on for ever. You aren't worried. You know it's there.
A tentative step. The gravel hardly stirs. Even so, you walk tip-toe, scared that you might tread on a sea anemone or a ray. Breathing out – had you not breathed out before? - you look up and watch the bubbles floating gently upwards, through the water, to break at the surface. It doesn't seem so very far above you; but water changes the perspective, you remember that. You remember, too, that the eye evolved to see in water. You never open your eyes underwater in the swimming pool, but you haven't thought to blink since you came through to this side.
It is surprisingly clear. (But why should it be so surprising?) The greenish tinge to the water has gone. The colours are brighter, the shadows deeper. The lurking shapes at the back have fins and feelers. Teeth. Panic. You mustn't panic. They know. They will smell it and they will come for you. You stand still as a rock, and the fear subsides. Nothing happens. They haven't seen you.
You can swim. Why not? Kicking off from the gravelly bed, you can glide between the weeds, the silvery shoals tickling your legs as they swim over you. They don't seem to notice you: perhaps to them you are only that ghost you saw from the other side.
(There was another side? Yes, of course there was.)
It's like flying might be, if you never had to be scared of falling. A gentle wriggle, and you are suspended horizontally, mid-water. You reach out a hand, push water back behind you. You feel hardly any resistance, and you move forward. A kick to go up, a nod to go down. You swoop and glide and the fish pass you lazily, silently.
Somewhere behind you, a tall shadow passes through the back of your vision, and five, perhaps ten, long seconds elapse before you remember what – who – it is. That frightens you, for a little moment, and you flail, thrashing through the water as you try to reach them. You flap your hands around. You want to attract their attention. Help help I'm drowning.
You aren't drowning. Are you?
You flap your hands faster. Waving. That's it. What did the sea say to the shore? Nothing, it just waved. That was meant to be funny. You can't remember why.
They can't see you. (Who can't see you?)
Another shadow. Danger. You flick around, dart away safe into the darkness.
There's a voice in your head. It speaks words that you only half understand. It says: if you find the glass again – when you find the glass again – you will be able to tap on it and pass back through into your own world of air and daylight.
Back through to where?