MiniGRAIL was a type of Resonant Mass Antenna,[1] which is a massive sphere that used to detect gravitational waves.

The strongest sources of gravitational waves are suspected to be compact objects such as neutron stars and black holes.

[4] When gravitation waves with frequencies around 3,000 Hz pass through the MiniGRAIL ball, it will vibrate with displacements on the order of 10−20 m.[5] For comparison, the cross-section of a single proton (the nucleus of a hydrogen atom), is 10−15 m (1 fm).

[2] The original antenna for the MiniGRAIL detector was a 68 cm diameter sphere made of an alloy of copper with 6% aluminium.

[7] The sphere is suspended from stainless steel cables to which springs and masses are attached to dampen vibrations.