Complex sentence: While on the bottom, Piccard and Walsh observed a number of small sole and flounder and claimed that the fish were swimming which would prove that at least some vertebrate life can withstand the extremes of pressure in the Earth's oceans.
Split into simpler sentences: While on the bottom, Piccard and Walsh observed a number of small sole and flounder. Their claim that the fish were swimming would prove that at least some vertebrate life can withstand the extremes of pressure in the Earth's oceans.