Leica Long Landscapes
The technique of shooting a landscape scene through a dense filter to extend the exposure time is quite popular with digital shooters these days. The same is possible with film if you use one that has a low enough ISO to start with - but you still have to use the very dark ND filter.
Start yourself out with the AGFA PAN 25 film - made by Maco Photo Products in Europe. A true 25 ISO, and extremely fine grain. You can reach the website of the manufacturer for development times here: