The foundation was covered in a solid shell known as a canopy, but for a Foundation Eater who had consumed many worlds along with their canopies, opening a small hole through which to enter was easy.

-Volume 3 page 88

天 盤 は 天蓋 という 強固 な 殻 に 覆わ れ て いる が、 数多 の 世界 を 天蓋 ごと 喰らっ て き た 天 盤 喰らい にとって、 極 小さな 穴 を 開け て 中 に 入る ぐらい の こと は 容易かっ た。

-Volume 3 page 110

“Those foundations are enclosed within boundaries known as canopies. That is an example of the “bubbles” I mentioned in my earlier explanation of the Sea.”

-Volume 3 pages 23-24