That’s an ambitious project, but before we start coding we should think about safety and legal permissions for an abandoned metro. Once that’s cleared, I can design a lightweight app that logs coordinates, takes photos, and uses a depth sensor or LIDAR for wall contours. I’d probably lean on a cross‑platform framework like Flutter for UI, integrate with ARCore/ARKit for real‑time mapping, and store data in a local SQLite DB that syncs to the cloud if needed. Let me know what tech stack you’re comfortable with, and I’ll sketch out the architecture.