Search for Magnetic Monopoles in Deep Ocean Deposits

High-field magnets and solid-state track detectors have been used in an endeavor to extract and observe magnetic monopoles from ferromanganese pavement that was deposited at the bottom of the North Atlantic Ocean during the last 16 million years. The failure to observe poles sets new and restrictive limits on the abundance of monopoles and upon their production cross sections. For proton-nucleon collisions, the new cross sections are ≤10^-42 cm² if the monopole mass is one proton mass m_p and ≤2×10^-34 cm² at 1000 m_p. Any magnetic charge up to 60 times Dirac's value of ℏc/2e could have been detected. The flux of monopoles reaching the ocean floor is less than 4×10^-18 (cm² sec)^-1 at the 90% confidence level, so that no portion of the cosmic-ray energy spectrum up to ≅10¹⁹ eV is composed dominantly of magnetic monopoles.