More than a century ago, an American astronomer named Seth Carlo Chandler discovered that, as the earth spun on its axis, it also wobbled. This wobble, now known as the Chandler wobble in honour of its discoverer, didn't disappear over time, as would have been expected if no further force reactivated it. The source of the continuing activating force has remained a mystery ever since - until now.
The Millennium Bridge across the Thames opened in June 2000 and was subsequently closed two days later because of the now-famous "wobbles". Given that, at any instant, two thousand people were suspended above a very dangerous river, this presented a serious problem.
In 1998, Goldbach's Conjecture was shown by computer to be true for even numbers up to 400,000,000,000,000. In addition, some progress has been made towards formally proving the conjecture. As of this year, mathematicians with Goldbach fever have some extra incentive for their labours.
Up until the late 1990s, astronomers couldn't be certain that any planets existed outside our solar system. These days, not only are astronomers confident that planets are out there, but new ones are being discovered all the time.