When it comes to finding synonyms for the phrase "hard to penetrate," certain words come to mind. One of them is impenetrable which means impossible to pass through or enter. Another synonym is impenetrable, which refers to something that is difficult to understand or that cannot be broken down. Other alternatives include resistant, unyielding, and tough, which all evoke the idea of something that is difficult to overcome. Indomitable and invincible are other options, where both words convey the idea of something that is unbeatable or untouchable. No matter which word you choose, they all point to something that is challenging and difficult to access.