The realtor called the staircase a "feature." It dropped past our new cellar, past a wine room nobody asked for, all the way down to a cold concrete floor. In Hollow Marsh, every house had one, each deeper than the last, like the town was competing. Our first night, Mrs. Edevane from next door knocked. "Whatever you do," she said, "don't go past the frost line."

Dad laughed off the warning and called Hollow Marsh "charmingly weird." That night I heard tapping under the floor. Slow, patient knocks, coming up through the concrete from somewhere far below the cold floor we'd already seen. Tap. Tap-tap. Like something asking to be let in.

I woke Dad and made him listen. The tapping stopped the second he sat up, like it was shy of grown-ups. "Old pipes," he yawned. But in the morning we found wet footprints leading up from the cellar door, across the kitchen, and stopping at the foot of Theo's bed. Theo was still asleep, smiling. From then on the footprints came every night, and each morning they stopped a little closer to where I slept.