**Moving along the familiar four dimensions of the multidimensional universe…**

Back in the 20s of the last century, the mathematician Theodor Kaluza and the physicist Oskar Klein noticed that adding another dimension to our four-dimensional space-time makes it possible to move forward in building a unified theory of the fundamental interactions known at that time: the gravitational and electromagnetic interactions. They assumed that the additional, the fifth dimension is folded into a circle so that the corresponding coordinate can vary from zero to the length of the circle’s circumference. A very small radius of the circle would explain the fact that the extra dimension could not be seen by us. Afterwords, a quantity of schemes have been devised in which there are several extra dimensions that, in addition, can be large or even infinite while remaining almost invisible without special experiments.