Karlo, located in Secret Valley, was a NCO Railroad station that was established in 1899. The post office was originally known as Secret, but the name was changed to Karlo in 1911. On December 1, 1925 postal authorities suddenly abandoned the Karlo Post Office, without consulting the residents. The residents were upset to say the least, for they now had to travel seventeen miles south to Wendel to pick up their mail. They petitioned to get the post office re-established, in which they succeeded. On December 26, 1925 the Karlo Post Office was back in business. The post office was finally closed in 1934 and made part of the Rural Free Delivery system.