The Sunken Stair Road

The old Imperium road that drowns and surfaces island to island across the Mirrenmere — the mere's only dry thread at the slack tide, gone at the flood.

The Imperials cut a paved causeway across the valley before the sea came; when the valley drowned, the road went with it except at the ridgeline crossings. Now it surfaces at low water — a stone road walking into and out of green water, island to island — and vanishes for hours when the tide is wrong. Miss the window and an island you can't swim off becomes your home for half a day.