Is it a condition to clearly see the impurity to rule on its presence on the ground, or can it be present yet not clear, and one is held accountable for it?

According to many scholars, impurity transfers if the impurity itself is wet, or if the ground that was trodden upon is wet or damp. It does not transfer if both are dry. Doubt about its transfer negates its transfer. The ground and anything connected to it become pure by drying, according to many scholars.