The written word revolutionized the diffusion and accumulation of knowledgeknowledge, leading to the invention of the book. The text of an average book contains about one million bytes of information (1 MB). In a magnetic memory, data are recorded as tiny magnetized regions on a recording medium, in magnetic hard disks (HDDsHDD (Hard Disk Drive)), tapes, and magnetic stripemagnetic stripe cards. In the first magnetic tapetape, magnetic system (1951), the data densitydata density was about 100 million times smaller than today’s. The data density in HDDs has increased almost a thousand million times since 1956. HDDs have recently incorporated energy-assisted magnetic recording (EAMR)Energy-Assisted Magnetic Recording (EAMR); also, new HDDs may use superposed tracks. The total information produced worldwide per year will reach about 2 × 1024 bytes in 2035. A pile of books containing this amount of information will be 40,000,000,000 thousand km high! This total illustrates the information explosioninformation explosion, one of the characteristic aspects of our era. In 2025, the information contained in all the data centers where information is stored reaches 2 × 1023 bytes. Magnetic storage is, therefore, the primary tool for preserving humankind’s memory.