The title is "not even" clickbait, just very confusing.

"The unique complex contains more than 1,300 square feet of underground space, including tunnels and storage rooms - and even a hand-built railway."

(Edit: I mean the original article, not the submission)

sorry for the capital M … I have fixed in the title to 15m as in metres

that’s ~ 45 ft underground, I believe … perhaps not that impressive, but quite funny since I would hesitate to undermine my own house and surely this makes it worth less