“Five stories?” Gauche laughter catches in my throat. If I couldn’t handle the metro, how will I handle centuries of crypt tunnels? I place my weight on my back heel. “I’d love for you to tell me about them here.”

Seb looks at me. “This is why we came. You did not realize they were so deep?”

His tone is flat, but it couldn’t carry more judgment than what I already feel; he’s right. I should go down—to glean even a second of communion with my spiritually minded sister, to see whether she left another clue behind. With a little more than a day to find her before my flight home, this is the best next step I have. A way to understand the person my sister became, knowing she isn’t lying on a steel slab. I take a deep breath. “Okay. Let’s go.”

“You are sure?”

“Yes. Before I change my mind.”

Past the ticket kiosk and inside its slim corridor, framed illustrations tell the history of rock quarries turned catacombs. While the quarries were leveraged for raw materials, the time came when the Black Plague accelerated the need for new burial ground. Several bouts of the disease struck the city, and existing cemeteries in the center of Paris dating back to the first millennium couldn’t handle the strain—the underground walls of adjacent buildings ruptured, and bodies fell into neighboring basements. Seeing an opportunity, city leaders decided certain areas of the tunnels would serve as new burial sites for old bones, and the larger cemeteries’ remains were relocated. From mining pits that evolved into structured tunnels, the Black Plague turned the underground matrix into mass graves.

As we begin the descent on a narrow spiral staircase, Seb recounts political details from the seventeenth to the nineteenth centuries. He speaks with more vigor than I’ve heard from him yet.

I nod through clenched teeth as he moves on to the prehistoric lakes of Paris and how they created the dank smell invading my nostrils now. With no sunlight or source of heat here, the chill is arresting. Placing my feet on each slender, spiraling step becomes a lesson in endurance and balance, when I just want to hunch over and shiver. The thin fabric of my hoodie provides all the insulation of tissue paper.