To possess or have access to something that will yield a very large amount of money, especially when that thing has yet to be profited from fully. If your social networking site is attracting millions of new users every month, then you guys are definitely sitting on a gold mine. With its vast stores of untapped crude oil, the small nation state is sitting on a veritable gold mine, but so far it has refused to allow any companies to drill on its land. Each one of those paintings would sell for hundreds of thousands of dollars. You can't just sit on a gold mine like that and not take advantage of it!