There's a clear dead end,
but it's one I can pass,
for I have no body,
hold no shape, bear no mass.
I float down the tunnel,
a maze of brown and amber,
away from the daylight,
into a warm, darkened chamber.
I nestle in the branches,
every cove, every niche.
Like a slug, I leave my mark,
and in a breath I'm set free.

Who am I?