The experiments were carried out by Thomas Udem from the Max-Planck Institute for Quantum Optics in Garching and colleagues. Udem and colleagues placed four atomic clocks - one rubidium clock, two caesium clocks and a hydrogen maser - in an air conditioned basement at Wessling in Germany and recorded their time difference every 4 seconds between the 3 and 23 August. To eliminate all possible external effects on the clocks, the team also monitored the background magnetic field, variations in temperature, power lines, and atmospheric pressure. Once these factors and the natural drift in the precision of the clocks were taken into account, no gravitational effects were observed.