The Gravitron was a gravity-manipulating device that was installed on a base on the Moon around the 2050s to control the tides on Earth, so that they in turn could control the weather. It was very delicate: five units off-centre and it would lift half of London into space. The powerpack powering the Gravitron was thermonuclear with an inner temperature of four million degrees.
It produced very intense sonic fields. Without protective helmets, the operators inside the Gravitron room would go insane in as few as twelve hours.
In 2070, the Telosian Cybermen infiltrated the moonbase and tried to take control of the Gravitron to use its gravitational effects to kill all life on Earth and eliminate it as a threat. However, they failed. (TV: The Moonbase)