From Denfert-Rochereau RER B to Lozere
(+ Climbing the hill to Polytechnique)
First follow the directions to arrive at Lozere (a railway stop of RER B)
Then, take the underground tunnel to exit the other way of the railway station and climb the hill until you enter the campus of Ecole Polytechnique.
Here is a sequence of snapshots to guide you. There are indication panels all the way.
All images, copyright Frank Nielsen, 2008.