The trio are taking part in the Summit on the Summit expedition in an effort to put the spotlight on the global water crisis, and raise money for clean water programs.
People.com reported the stars spent several days visiting a village near Arusha, Tanzania, before they started their climb Thursday.
"Meeting the people and community in Tanzania … has been an invaluable and eye-opening experience for me," the magazine's Web site quoted Biel as saying Tuesday. "My hope is that others will be inspired to find ways that they can help; starting simply in their homes and continuing to other parts of the world. We are all in this together."