Game Help:Sims 3 Dive Wells
From SimsWiki
some dive wells contain tunnels inside them at the bottom you will be notifide if your sim finds a tunnel when a popup shows and ask do you want go through the tunnel or go back to the surface. tunnels usually lead to another room in the tomb/building and those rooms will contain some things wich your sim might need durning their adventure or if they are just exploring the tombs.
TIP:if your sim is going to dive in a well it would be better if they chaned into their swim wear so that once they get out of the well they wont be drenched in water and feel uncomfortable