Before you enter the grotto itself, you can browse through a small exhibition, explaining a bit about the history of the place and the restoration measures that have been taken over the years. There are numerous exhibits of other things that have been made with shells, from bowls across picture frames to religious altars.
When you're ready, you descend a small staircase into a dimly lit tunnel with roughly made walls. And then you see the shells. And you marvel at the patience and artistic virtue of whoever decided to create this underground mosaic. There are shell roses, and shell hearts, and shell stars and they are all really, really beautiful. There's even a little cupola lined with shells, opening up to a skylight.
At the end of the tunnel, there is something like an altar chamber, where secret meetings, seances and Halloween parties have taken place across the years.
As the grotto was lit by gas lamps through most of its existence, the mosaics are covered with carbon residue and are a monochrome scheme of greys and blacks. But in the exhibition room you can see how colourful the shells would have looked in the early days of the grotto. They would have been all different hues of white and yellow and blue.
If you are interested in seeing the grotto for yourself or finding out more about it, you should have a look at the Shell Grotto website, which will give you a wealth of additional information