You're on an island and desperately need to get off alive. There are 2 caves in front of you. One cave tunnels under the ocean somehow and leads to safety, and the other cave leads to certain death. There are 2 people standing in front of you. You know that one always tells the truth, and one always lies, but you can't tell which person is which. Both people know which way leads to safety and which one to certain death. You have one question to ask one of them in order to figure out how to get off the island alive. What is the question?