“Thy way was in the sea, and Thy path in the great waters, and Thy footsteps were not known.” – Psalms 77:20, JPS 1917 Tanach
After the Children of Israel crossed through the Sea of Reeds, the sea returned to its original state. As a result, the actual footprints of the Children of Israel were covered up by the waters. Therefore, there was no visible reminder left, in regard to the great miracle that was done for our ancestors. It’s as if G-d’s footprint, so to speak, were no longer recognizable, in terms of His guiding hand that figuratively speaking, parted the sea. The same may be true today as well, for all of the countless miracles, whereof we do not even attribute His hand as having had any influence. Yet, if we knew the truth of G-d’s involvement in our lives, then we would abound with gratitude for all that He has done for us as individuals, and also as a nation.