well I (the submitter) messed up and wrote 15M in the submission title (note the cap) in error just to add to the confusion (now fixed, soz.) … I thought “best” just to repeat the article title

No worries, I take issue with the person who concocted the original title. I got here after the capital-M change and it was still confusing that they didn't mention the space that was dug out at this depth, instead paint a picture of a system of tunnels under a normal house(?)