[szszrk]

Well the article claims cave is pitch dark, so I guess the invisible part is granted anyway.

[kijin]

Cave-dwelling animals don't just crash into walls all the time, so by "visible" I mean large enough and dense enough to be recognized as an obstacle in whatever navigation system they use.

[hnlmorg]

They do rest on walls though.

[thaumasiotes]

In this case every part of the wall is actually just a spiderweb.