Malbim (Bereishis 7:23) All existence on earth was blotted out While life was blotted out, many of their bones which were as strong as iron bars and did not disintegrate. Most of the carcasses were carried by the flood waters into deep crevices. They were thus buried deep in the earth. Consequentlt when Noach left the Ark, he found no trace of the carcasses of animals or the big bones of of the large animals or giants that existed before the Flood. The testimony of G-d is reliable and enlightens the fool. And is the response to the scholarly geologists who dig in the depths of the Earth where they find the large bones of giants and large animals. That were lost from the earth and have not existed after the days of the flood, They claim this is proof that the world is very old. That is because that the world is composed of different materials and that different distinct depths there is a different type of earth and they calculate based on these layers the age of the bones. They believe that the world is thousands of years old and that there were already animals from ancient times before the time of our creation. And this is nonsense, because at the time of the flood the earth opened up because of the earthquakes and they overturned the layers bring the lower levels to the top and the top layers to the bottom as well as lowering the carcasses to the lower levels. This process of overturning the layers which continued for 150 years confounds the scientists . They should only be aware of the deeds and wonders of G-d as to how he obliterated the world and drowned them to the depths.
Netziv (Bereishis 7:23) All existence on earth was blotted out That means that their bodies were removed from the surface of the world. But many of the bodies were covered with dirt by the Flood but remained intact. In fact these are source of the bones that have been found by those who dig in the dirt and find bones of creatures which don’t exist now in the world. The many scientist conclude from them that there must have been a different world in existence proior to the creation of this world. In fact in Bereishis Rabbah it clearly states that there were worlds created before this one which G-d destroyed. This idea is also found in the Zohar. Nevertheless I find this problematic as we see in Shemos Rabbah that what came before is not significant and nothing remained of them. I think that these bones are from those that existed before the Flood and no longer exist. Even though they flourished in a different climate but they deviated from their previous life prior to the Flood and moved to a different place. They continued to exist by breeding with different species and produced strange offspring like a horse breeds with a camel. It is the result of Divine supervision that these bones remain so that the contemporary generation can learn the secrets of Nature and the Glory of G-d. .