More than 60,000 people have queued for a glimpse inside the particle accelerator being built by the European Organization for Nuclear Research (Cern) in Geneva.
The open weekend was a one-off chance to see the Large Hadron Collider (LHC) before it begins operating in June.
The instrument will be used by physicists to explore the make-up of "dark matter" – the invisible mass of energy that is believed to make up 96 per cent of the universe. Once it is set in motion only technicians and physicists will have access.
The accelerator will smash particles together at high speeds within a 27-kilometre tunnel. Scientists hope it will recreate the conditions which existed after the Big Bang, the theory that the universe expanded from an extremely dense and hot state some 13.7 billion years ago and continues to expand today.
The LHC tunnel runs between Lake Geneva and the Jura mountain range and generates temperatures colder than deep space. It has been in construction for 15 years.
The SFr6 billion ($5.95 billion) accelerator will be officially opened at the end of October by the heads of 20 member states of Cern.
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