From St Chapelle, it is only a few short blocks to Notre Dame cathedral. It was threatening to rain, but never actually got going, which was great. We entered the massive cathedral and did a loop of the interior. There was a mass going on, and signs everywhere directing tourists to a loop around the perimeter, and parishioners to the center where the pews were. It was pretty dark inside, and the sounds of the chanting and organ playing were almost surreal. As non-Catholics (and non-French speakers), we didn't really understand what was going on with the service, but we were quiet and respectful and appreciated the pageantry of the sounds and the setting.