The U.S. Department of Energy still has the two most powerful supercomputers in the world, including Summit at Oak Ridge National Laboratory, according to a semiannual list released Monday.

It’s the fourth time in the past two years that Summit, an IBM-built supercomputer, has been number one on the TOP500 list of of the world’s most powerful supercomputers.

The United States displaced China at the top of the list last year, in June. Two years ago, in November 2017, China had the top two systems in the world.

Summit debuted at number one in June 2018. That was the first time since 2012 that the United States had the most powerful supercomputer in the world. Summit retained the top spot in November 2018 and again in June 2019.

The Sierra supercomputer at Lawrence Livermore National Laboratory in Livermore, California, climbed to number two from number three in November 2018. It remained at number two on the June list and again on the list released Monday, meaning it’s been number two on three versions of the list in the past two years.

Summit and Sierra are both IBM-built supercomputers that use Power9 central processing units (CPUs) and NVIDIA Tesla V100 graphics processing units (GPUs).

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