Imo, she uses the forgotten tunnel with “mosaic ceilings painted tiles on the wall” to symbolize a seemingly mundane exterior (a forgotten tunnel) with a beautiful interior.

She uses this as a metaphor for herself. At the core, Lana is beautiful like the inside of the tunnel — but the exterior is what people see (or even what she sees). Lana feels like she has figurative “walls” that distract from her true self — maybe her “man-made” beauty, aesthetics, self-consciousness, etc etc.

But at the core, without all the distractions, she sees beauty within herself. She likens her body to the exterior of the tunnel, and her soul to the interior. She feels like her body marred her soul (the interior), maybe her curated image in the public distracted people from Lana’s inner beauty, something that she searches for in this song.

This song, to me, is a manifesto of sorts for Lana to continue to find her inner beauty rather than focus on her outer beauty. She doesn’t want her lover to forget her… or at least she doesn’t want her lover to remember her for her walls, her “bad” qualities. It’s a very sad song, and there’s no way I can infer exactly what she means. But I hope this rambling helps lol.