Translation of "Impenetrable" in Spanish

This seems to be an impenetrable mystery,

Parece un misterio impenetrable,

Reminiscent of the green border, which became impenetrable

recuerdan el borde verde, que se volvió impenetrable

We can come up with solutions to previously impenetrable problems.

Así, podemos encontrar soluciones a problemas que creíamos irresolubles.

Tom stared into the impenetrable blackness of the night sky.

Tom miraba la oscuridad impenetrable del cielo nocturno.