they remain mysterious

When I walk by the ponds at work, I often hear a squeak followed by a splash - frogs jumping into the water upon noticing me. No matter how quietly I walk or how intently I look, I never see them; I only see the rippling water after the squeak!splash!. I don't know how big or small they are or what they look like. I assume they are frogs though.

One day while leaving work last week, it was already dusk. Outside, I saw a large frog hopping across the sidewalk! It stopped a moment while I gleefully watched it. But it was too dark to see any details; I only saw its general shape. It hopped away in a very frog-like manner.