The rays that have very short wavelengths and penetrate almost any kind of metal are gamma rays.

Here's why:

* Short Wavelength: Gamma rays have the shortest wavelengths in the electromagnetic spectrum, ranging from less than 10 picometers (10^-12 meters) down to extremely short wavelengths.

* High Energy: Due to their short wavelengths, gamma rays carry very high energy.

* Penetrating Power: This high energy allows them to penetrate deeply into matter, including metals. Even thick layers of lead or concrete are only partially effective at blocking them.

Important Note: Gamma rays are a form of ionizing radiation, meaning they can damage living tissue. This makes them potentially hazardous.