The most surreptitious way of cracking a safe is to manipulate the lock to obtain the combination without actually damaging the safe. Some rotary combination locks can be manipulated by feel or sound. More sophisticated locks use wheels made from lightweight and soft materials such as nylon, which reduce this vulnerability.
A safe may be compromised surprisingly often by simply guessing the combination. Manufactured safes often come with a manufacturer-set combination. This combination (known as a try-out combination) is designed to allow the owner initial access to the safe. Combinations are also unwittingly compromised by the owner having the lock set to easy-to-guess combinations, such as a birthdate.
A number of companies and groups have developed autodialing machines to open safes. Such machines are usually specific to a particular type of lock and must cycle through thousands of combinations to open a device.
While most safes are hard to open, most are susceptible to compromise by drilling or other physical methods. Manufacturers publish drill-point diagrams for specific models of safes. Other methods generally involve damaging the safe so that it is no longer functional. These methods may involve explosives or other devices to inflict severe force.
You will need to use a mid-20th century method to crack The Diamond Safe.
And watch out for pesky security guards!