The Roman military site of Aquincum near Budapest in Hungary is already known for five temples of Mithras. A housing development in the area has uncovered a sixth temple, discovered in the summer and just now reported by Oliver Kovács in a Hungarian archaeological website, Muemlekem.hu. There are a number of photos with the article! More excavation is planned for the spring, but we must hope that the site is preserved. There is some doubt whether the Hungarian authorities have done enough to ensure this.
Mithras expert Dr T. Csaba Szabó has written more about this here.