There's a kind of peace here in the Tunnels that can't be found anywhere else Catherine has ever traveled, and she cannot explain why. The easy answer is to say her tranquility comes from being here with the man she loves and the son she adores beyond words. Their presence brings her a surfeit of joys, and anyone could be content in that tremendous a glow. But she knows that as much as she loves Vincent and Jacob, it's more than just the two golden centers of her life making her feel this way.
Candle smoke tickled her nose the first time she woke up here, groggy from her wounds. Now the burning tallow is the scent of home. Beyond their chamber is the ever-present sound of the tapping pipes, and far above, the distant rumble of subway cars. The noises soothe her into sleep each night along with the familiar rise and fall of Vincent's breath in her ear. She brought warm clothes with her when she made her choice, not the fashionable suits or dresses of the life she dropped behind her, and these soft, worn fabrics embrace her as tenderly as a hug.
Above, there are millions of electric lights dazzling a city famous for its lack of sleep. Below, there are yellow flames and flickering lamps. Above, Jacob could be attending a prestigious preschool to put him on the road for excellent schools. Below, he is cared for by a dozen friends and family as well as his parents, who read him classic literature and play with him and love him more than the world. Catherine surrendered her life Above a week after she found out he was on the way, spending her last few months before she started to show by snipping her ties to that old life. She's never regretted her decision for a single moment.
The Tunnels are her home. This peaceful world is where she has always belonged. As she watches Jacob play with his favorite rag doll, she can't imagine wanting anything more.