In Jules Verne, Voyage au centre de la Terre, chapter 32, Axel seems to believe that algae can grow 12000 feet (4000 meters) under the sea. This is an order of magnitude wrong. Why would Axel believe that?
Quoting the text from chapter 32,
Vers midi, des algues immenses vinrent onduler à la surface des flots. Je connaissais la puissance végétative de ces plantes, qui rampent à une profondeur de plus de douze mille pieds au fond des mers, se reproduisent sous une pression de près de quatre cents atmosphères et forment souvent des bancs assez considérables pour entraver la marche des navires ; mais jamais, je crois, algues ne furent plus gigantesques que celles de la mer Lidenbrock.
(For an English translation, see chapter 32 in translation (1877) by Frederick Amadeus Malleson , also same translation.)
Note that the way the sentence is written, the 12000 feet definitely refers to the water depth in a sea, because it matches with the nearly 400 atmosphere pressure in the next clause. In particular, it does not refer to the depth of the surface of the Lidenbrock sea under the surface of Earth: apart from the grammar, that would not match the (mistaken) in-character beliefs of Axel, who thought they were between 110000 and 160000 meters under Earth's surfce. The numbers in that particular passage could be approximations though, which is why I'm asking about 3500 meters in the title rather than 4000 meters.