It’s a marvelously erratic move for both Fincher and Disney, and according to the post from THR Fincher approached Disney with a view to direct a family friendly tentpole movie and 20,000 Under the Sea appears to be that project. Scott Z. Burns is on the case with the script, his previous work includes The Bourne Ultimatum and The Informant! and there’s every chance this will have nothing to do with the abandoned McG Captain Nemo movie which recently fell from grace, allowing Fincher to pick it up.
The 1954 version of the film had Kirk Douglas, Peter Lorre and the sublime James Mason and was a delicious take on the story, and is usually the source of the first images conjured up by the title, so Fincher will have to work hard to crowbar our preconceptions from the property. As this is still in the development stages there seems to be plenty of time for Fincher to complete the, arguably unnecessary, Girl with Dragon Tattoo adaptation.