impenetrable (adj.)
- ambiguous
- arcane
- cabalistic
- close
- close-knit
- compact
- compressed
- concentrated
- concrete
- congested
- consolidated
- crowded
- dense
- exuberant
- firm
- flourishing
- gluey
- gross
- hard
- heavy
- impassable
- impermeable
- impervious
- impregnable
- inaccessible
- inarticulate
- incognizable
- incoherent
- incomprehensible
- incontestable
- indomitable
- inexpugnable
- inscrutable
- insuperable
- invincible
- inviolable
- invulnerable
- irresistible
- jam-packed
- jammed
- jungly
- lush
- luxuriant
- massive
- mystic
- nonporous
- numinous
- overgrown
- overpowering
- overwhelming
- packed
- rambling
- rank
- resistless
- riotous
- serried
- solid
- substantial
- thick
- thickset
- un-come-at-able
- unaccessible
- unaccountable
- unapproachable
- unassailable
- unattainable
- unavailable
- unbeatable
- unconnected
- unconquerable
- undiscoverable
- unfathomable
- unintelligible
- unknowable
- unobtainable
- unpassable
- unprocurable
- unreachable
- unsubduable
- unsurmountable
- unyielding
- viscid
- viscous
- weedy
