The four men were covered in black equipment from head to toe. Black hats, black coats, black shoes, and even black gloves on their hands.

Moreover, their black outfits were very different from any ordinary black fabric. Most black fabrics were just black in color, but the color of the material of their accessories was very thorough, like a black hole.

In truth, the reason for such an effect was that a layer of ultra-black material with a very low light refractive index was attached to the outfit, which was called carbon nanotubes. Its reflectivity to light was only 0.035%, that was 3.5 ten thousandths. It was the blackest material created by mankind so far.

In a poorly lit environment, the naked eye could barely see clothing that was coated with this kind of material, hence, this high-tech material was an excellent tool for ninjas who were good at concealing their whereabouts. Wearing this kind of clothing allowed them to increase their ability to conceal their location greatly.

A