OP almost certainly isn't in the UK, but even if so, you spread it around numerous banks and different asset categories (stocks, gold, bonds) rather than risk it all in a safe. Even just dumping £1m into an NS&I Income Bond would keep it pretty safe.

I'd personally diversify as much as possible and buy into a variety of funds covering most major geographic areas (e.g. a fund that tracks the Dow, one for the Asian markets, and so on.)