dense
incomprehensible
thick
uncomprehensible
Greek to one
ambiguous
arcane
beyond reach
beyond understanding
cabalistic
close
close-knit
close-textured
close-woven
closed forever to
closed to
compact
compacted
compressed
concentrated
concrete
condensed
congested
consolidated
crammed
crammed full
crowded
exuberant
firm
flourishing
gluey
gross
hard
heavy
holeproof
impassable
impermeable
impervious
impregnable
inaccessible
inarticulate
incognizable
incoherent
incontestable
indomitable
inexpugnable
inscrutable
insuperable
invincible
inviolable
invulnerable
featureless
icy
impassive
invisible
leafy
misty
murky
opaque
sombre
thorny
unbroken
unfathomable
penetrable