To access the OpenCaching.com API you need to developer key. A development key is a 16-character string consisting of upper case letters, lower case letters, and numbers. Each application or website that is going to be accessing the API needs to get its own key.
Email email@example.com and include the following information.
Your API requests must include your developer key in the HTTP Authorization header or in an Authorization parameter in the query string. For example: http://www.opencaching.com/api/geocache?center=38.642,94.754&limit=5&Authorization=your_developer_key.