impenetrable

"During the war, the city became a fortress against enemy attacks. "

"My daughter's teacher is very kind and helpful. "

"The waters of the lake were transparent until the factory dumped waste there."

"The student received a small quantum of extra credit for participating in class. "

"The darkness of the room made it difficult to see."

"The poorly guarded border was easily penetrable. "

"While incomprehensibility is often used to describe advanced physics, the doctor's handwriting was pure incomprehensibility to the pharmacist. "

"The curtains were translucent, letting the sunlight filter through but softening the harsh glare. "

"The lake was fathomless, and no one knew what lay at its deepest point. "

"Example Sentence: "After years of studying the ancient symbols, the archaeologist still considered the Rosetta Stone an inscrutable." "

"My teacher explained the new math concept, and I understood it perfectly. "

"The explanation was long and drawn-out."