By Emese Bartha and Chiara Albanese 

Europe's plunging borrowing costs marked two new milestones on Wednesday, with Switzerland becoming the first country ever to issue 10-year debt that gives investors a yield under 0%, and Mexico lining up a rare deal to borrow euros that it will repay a century from now.

Bond prices across Europe have rocketed this year in response to the European Central Bank's massive stimulus package, delivering bumper returns for existing bondholders and cheap deals for borrowers, but crushing yields for new investors.

Several European countries inside and outside the eurozone have sold government debt with up to five years of maturity at negative yields, which means investors effectively pay for the privilege of buying it. But no other country has previously stretched this out as long as 10 years.

Meanwhile, Mexico is thought to be the first borrower from outside the euro area to issue 100-year euro debt.

"Given the prospect of sustained monetary policy easing by the ECB for the foreseeable future, issuing long-term paper denominated in euros makes sense from a borrower's perspective. We expect this trend to strengthen," said Salman Ahmed, global fixed-income strategist at Lombard Odier Investment Managers.

Switzerland sold a total of 377.9 million Swiss francs (about $391 million) of bonds maturing in 2025 and 2049. On the 10-year slice, the yield was -0.055%, compared with 0.011% on its most recent similar bond two months ago.

In the post-issuance secondary market, Swiss bonds maturing up to 11 years in the future already trade with yields under 0%. But such low yields at the initial point of sale "illustrate well the world we live in," said Jan von Gerich, chief strategist at Nordea, referring to collapsing yields on debt amid widespread stimulus from central banks around the world, a trend enhanced by the ECB's latest bond-market foray.

In January, Switzerland's central bank scrapped its upper limit on the value of the franc and cut deposit rates to -0.75%. Swiss bonds are likely to remain attractive to investors as long as yields stand above that level.

"The combination of deflationary fears and aggressive central-bank action has caused investors to accept the reality of negative-yield bonds," said Jeffrey Sica, chief investment officer of U.S.-based Circle Squared Alternative Investments. An auction with a negative yield "signals a lack of confidence from investors that the economy will be growing in the short term."

Mexico's deal, which is set to wrap up later Wednesday, is expected to give investors a yield of about 4.5%. The country has previously sold century bonds in sterling and in dollars.

"With euro rates in negative terrain, Mexican sovereign risk looks pretty attractive, particularly in these terms," said Marco Oviedo, chief economist for Mexico at Barclays. He expects a healthy demand from European investors.

Borrowers from around the world, including big names such as Berkshire Hathaway Inc and Coca-Cola Co, have been drawn to the allure of cheap funding in euros this year. Local borrowers have also feasted on the cheapest ever deals. French utility GDF Suez SA issued bonds in March with no guaranteed regular payments to investors at all.

Write to Emese Bartha at emese.bartha@wsj.com and Chiara Albanese at chiara.albanese@wsj.com

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