Black and grey carp and waves on a thigh

Her name was Mireille, though the dockers called her the Grey Lady, and she had scales like hammered pewter and a head that could have been a dragon's if you squinted through the spray. Old Ottoline Pike kept her in the cistern under the drowned bank, feeding her crusts and bent nails of salt. When the waters came in the autumn after the Tilt, Ottoline was gone, and Mireille was left circling the flooded reading room with nobody to tell her where the air was.

So she went looking for it. Up through the sunken lobby, past the clock that had stopped at a quarter to nine, through the spiral of the Mercer tower where the waves folded over each other like cloth being shaken out. On every landing the water thinned and the cloud came down to meet her, grey and cold, and she climbed it the way you'd climb a rope. The boy who kept the beacon on the roof said he heard her long before he saw her, a slap of tail against iron, slow and patient as a clock.

She came over the parapet at first light and lay her great length across the leads, gills working, while the last lamp guttered out beside her. The tide turned that morning. It has never once come all the way back up the stairs since, and that is why you can still climb to the roof of Harrowgate dry-shod. Mind the third step. She left her scales on it.