The downward march of long-term bond yields in 2016, which have sent the entire Swiss yield curve into negative territory, has been a wonder to behold.

For investors who bet on super-low yields to go even lower this year, the returns in the first half of the year have been stunning.

The above chart shows what the capital appreciation generated in local-currency terms from investing in a Canadian bond maturing in December 2045, a U.S. Treasury maturing in November 2045, a Swiss bond maturing in January 2049, a U.K. gilt maturing in January 2045, a German bund maturing in August 2046, and a Japanese bond maturing in December 2045.

The standout performer, the Japanese long-term bond, has rallied by more than 30 percent year-to-date.

By contrast, an investment in the S&P 500 at the start of 2016 would have garnered a total return of roughly 4 percent at the end of the first half.

As a caveat, these hypothetical returns on investments in long-term sovereign debt do not take into account coupon payments. But with yields at low-to-negative levels, capital appreciation and not coupon clipping, has been the driving force behind the total return of long bonds.