After closing time

An original description of an invented aquarium after its visitors have left.

Once the doors had been locked, the aquarium became a building of whispers. Water slipped through filters behind the walls, and bubbles climbed the glass with the patience of rain travelling upwards. The empty entrance hall still shone beneath its cheerful painted fish.

Beyond it, the tanks supplied their own dim light. A shoal turned together, flashing silver and then vanishing into grey. In the largest tank, a ray passed over the sand so smoothly that its shadow seemed to move before it did. Nothing hurried, yet nothing was entirely still.

I followed the cleaning trolley into the tunnel beneath the deep-water tank. Above me, the dark shape of a shark appeared without warning. It crossed the curved roof in silence, blotting out the green light for a moment, and disappeared behind a wall of rock. The water pressed against the glass on every side.

A paper crown dropped by a child lay beside the tunnel exit. I placed it on the trolley and walked more quickly towards the bright staff corridor.

How does the writer make the empty aquarium seem both peaceful and unsettling?