ATLAS (A Toroidal LHC Apparatus) is one of four detectors receiving high-energy particles from the LHC at CERN.
The detector is shaped like a barrel lying on its side in a cavern about 100 metres below the ground near the main CERN site near Meyrin, Switerland. The detector itself is the size of a cathedral, about 46 metres long and 25 metres in diameter. It total it weighs about 7,000 tonnes.

About 3,000 physicists from over 175 institutions in 38 countries are involved with experiments using the detector. It was one of the two experiments which discovered the Higgs boson in July 2012.

References

Main ATLAS website: http://www.atlas.ch/

eTour of the accelerator: http://www.atlas.ch/etours_accel/index.html