You originally asked two questions which I have now edited down to one according to enotes policy. Clearly the major symbol in this excellent short story is the tunnel itself, but I am going to look at the two different beaches depicted in the story and consider how they are used symbolically.
Key to working out how they are presented symbolically is to look at how the two beaches - where Jerry's mother is and the other beach where the tunnel is - are compared, and the kind of imagery that is used to compare them. Consider this quote describing the beach of the tunnel:
From where he was, high up among red-brown rocks, it was a scoop of moving bluish green fringed with white. As he went lower, he saw that it spread among small promontories and inlets of rough, sharp rock, and the crisping, lapping surface showed stains of purple and darker blue. Finally, as he ran sliding and scraping down the last few years, he saw an edge of white surf and the shallow, luminous movement of water over white sand and, beyond that, a solid, heavy blue.
And what about this one describing the "safe" beach of his mother:
There she was, a speck of yellow under an umbrella that looked like a slice of orange peel.
Clearly imagery is used to make the "dangerous" beach appear exciting, mysterious and scary, whilst more domesticated images are used to present the "safe" beach. It is highly symbolic therefore that Jerry feels drawn towards the more dangerous beach rather than the safer beach where his mother dwells.