This will mark the fourth year Mike Riggs has taken the plunge into Lake Tahoe’s frigid waters to raise money for the Special Olympics. Despite his experience, he’s still not quite sure what to expect. “Every year is a new surprise,” he said. “You never know what Mother Nature is going to bring you.” The annual Polar Plunge will be Saturday at Lake Tahoe’s Zephyr Cove. So far, $43,473 has been raised for Special Olympics Nevada. Participants, known as plungers, must raise at least $125 through pledges and have ranged in age from 7 to 70. Individuals are welcome, as …