Who is the antagonist of Tunneling to the Center of the Earth?

Kevin Wilson's Tunneling to the Center of the Earth does not have a proper villain and, indeed, the conflict seems minimal. There is no big bad guy standing in the way of the heroes. The three main characters are all recent college graduates who left university with "useless" degrees in niche topics like gender studies and Canadian history. They decide to dig tunnels in the ground in the wake of their graduation.
The antagonist would appear to be within the characters themselves. A psychiatrist later in the story claims the characters are trying to run away from reality by going underground and putting off the adult world represented by everything above ground.
If one does not accept this diagnosis (which the main character does not), the antagonist would be society itself. The protagonist's choosing to tunnel underground is viewed with disapproval by the adult figures above ground, who see it as running away. For the protagonist, the act of tunneling represents getting in touch with concrete reality. It is also an act of self-discovery, since he chooses to become a landscaper after the experience, an occupation which gives him fulfillment. However, this discovery would not have come about had the protagonist chosen to back out of the quest too early.