The term 'difficult to penetrate' can be described by two crossword solutions: 'MURKY' and 'DENSE'. MURKY is an adjective that means dark, gloomy, and obscure. It is often used to describe a situation or environment that is difficult to understand or make sense of. DENSE is an adjective that means thick, compact, and heavy. It is often used to describe something that is hard to penetrate or get through. Together, these two words accurately describe the term 'difficult to penetrate'.