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."

My little brother Theo didn't wait for warnings. By the time I turned around, he was already three steps down, giggling, counting each one out loud. "Seven, eight, nine," he sang. His voice echoed up wrong, like the stairs were much longer than they looked.

I yanked Theo up the stairs and slammed the door. He cried, then calmed down, then asked a question that froze me solid. "Why does the other Theo get to stay down there?" he said. "He looks just like me. He's still counting." I held him tight and didn't answer. But that night, and every night since, I hear two boys counting under the floor, and I'm too scared to check which one is mine.