This is one of the LHC's four main experiments. Atlas will search for new physics in the head-on collisions of protons that are accelerated to fantastic energies.
The Atlas detector is about 45m (148ft) long, more than 25m (82ft) tall, and weighs about 7,000 tonnes. It is about half as big as a Notre Dame Cathedral in Paris and weighs the same as the Eiffel Tower.
Atlas and CMS are the LHC's general purpose detectors that will go in search of the elusive Higgs boson, which gives other particles their mass.
Located at Meyrin in Switzerland, Atlas will also look for extra dimensions of space, microscopic black holes, and evidence for dark matter.
If all the data from Atlas was recorded, it would create a stack of CDs 137m (450ft) high every second.
The main feature of the Atlas detector is its enormous doughnut-shaped magnet system. This consists of eight 25m (82ft)-long superconducting magnet coils, which are arranged in a cylinder around the beam pipe which runs through the centre of the detector.
More than 1,700 scientists from 159 institutes in 37 countries work on the experiment.