Current experimental limits on the number of magnetic monopoles bound in matter are based on only a few dozen kilograms of material, and of the former experiments only a handful used superconducting-quantum-interference-device- (SQUID) based detection techniques. Furthermore, all previous searches for trapped monopoles used material formed or gathered from either the surface of the Earth or Moon, where ultramassive grand unified field theory monopoles would not be expected to stop. Using a new type of ultraefficient SQUID magnetometer which allows large volumes of room-temperature matter to pass directly through a superconducting loop, we examined a suite of high-pressure metamorphic rocks which had been buried at depths of up to 25 km and yet remained below the Curie temperature of the ferromagnetic minerals present. In addition, we also examined large volumes of manganese nodules and seawater, for a total mass of 643 kg of rock and 180 kg of seawater. No monopoles were found, suggesting that their cosmic abundance is either extremely low or they are not easily stopped or trapped by passage through a minimum of 25 km of the Earth’s crust. We suggest that the next best place to look for monopoles is in cometary dust.

• Received 5 August 1985

DOI:https://doi.org/10.1103/PhysRevA.33.1183