There have only been two times in the last 24 years where the U.S. has been edged out of the top spot of the world's most powerful supercomputers. Now is one of those times. "An upgrade to a Swiss supercomputer has bumped the U.S. Department of Energy's Cray XK7 to number four on the list rating these machines," reports the BBC. "The only other time the U.S. fell out of the top three was in 1996." The top two slots are occupied by Chinese supercomputers. From the report. The U.S. machine has been supplanted by Switzerland's Piz Daint system, which is installed at the country's national supercomputer center. The upgrade boosted its performance from 9.8 petaflops to 19.6. The machine is named after a peak in the Grison region of Switzerland. One petaflop is equal to one thousand trillion operations per second. A "flop" (floating point operation) can be thought of as a step in a calculation. The performance improvement meant it surpassed the 17.6 petaflop capacity of the DoE machine, located at the Oak Ridge National Laboratory in Tennessee. The U.S. is well represented lower down in the list, as currently half of all the machines in the top 10 of the list are based in North America. And the Oak Ridge National Laboratory looks set to return to the top three later this year, when its Summit supercomputer comes online. This is expected to have a peak performance of more than 100 petaflops.