| Answer | Crossword Clue |
|---|---|
| DENSEST | Hardest to penetrate |
| MURKY | Difficult to penetrate |
| DENSER | Harder to penetrate |
| DENSE | Hard to penetrate |
| ARMOR | It's hard to penetrate |
| CLADS | Armor- -- (tough-to-penetrate warships) |
| CLADS | Armor-... (tough-to-penetrate warships) |
