What was Atlantis and what did Professor Aronnax think about it?

The sequence that you are looking for can be found in part 2, chapter 9. Nemo and Aronnax are out exploring the ocean floor, and they come across some stones that don't look natural. Nemo and Aronnax continue until they reach a summit, and Aronnax sees a lava-spewing volcano. Beyond that, he sees an ancient city. Nemo writes on a rock the word "ATLANTIS." Atlantis is an ancient, fictional city and is the stuff of legends. Verne's narration of Aronnax's thoughts actually does a really nice job of explaining just how important Atlantis has been in terms of historical texts. Supposedly, it was a city and civilization of great learning and technology; however, it was catastrophically destroyed and lost forever.

Atlantis, that ancient land of Meropis mentioned by the historian Theopompus; Plato's Atlantis; the continent whose very existence has been denied by such philosophers and scientists as Origen, Porphyry, Iamblichus, d'Anville, Malte-Brun, and Humboldt, who entered its disappearance in the ledger of myths and folk tales; the country whose reality has nevertheless been accepted by such other thinkers as Posidonius, Pliny, Ammianus Marcellinus, Tertullian, Engel, Scherer, Tournefort, Buffon, and d'Avezac; I had this land right under my eyes, furnishing its own unimpeachable evidence of the catastrophe that had overtaken it! So this was the submerged region that had existed outside Europe, Asia, and Libya, beyond the Pillars of Hercules, home of those powerful Atlantean people against whom ancient Greece had waged its earliest wars!

Aronnax's reaction is predictable. He is inspired by the sight and even feels that he is walking on and seeing hallowed ground. Aronnax then begins contemplating basic plate tectonics and considering the possibility that the ruins might some day be lifted off of the ocean floor.

One day perhaps, some volcanic phenomenon will bring these sunken ruins back to the surface of the waves!

Nemo then urges Aronnax back to the submarine, and Aronnax is disappointed with his short time in the amazing ruins.