impenetrable
impenetrable mystery

muse
to muse on the mystery of mortal life

mystery
...cloaked in mystery / to be shrouded in

plumb
to plumb the depths of a mystery

rest
the affair rests a mystery

shroud
to be wrapped in a shroud of mystery

shrouded
shrouded in mystery

unfathomed
unfathomed mystery

unsolved
an unsolved mystery

veiled
to be veiled in mystery

wrap
to wrap (up) in (a) mystery

wrapped
wrapped in mystery