On the trail of Albert Einstein's last mystery
It is 100 years since Albert Einstein submitted his Theory of General Relativity, a pillar of modern physics that has transformed our understanding of space, time and gravity.
Most of the predictions that he made back in 1915 have subsequently been confirmed. But one remains elusive - the direct detection of gravitational waves.
Now, a number of observatories around the world are trying to get a glimpse of these phenomena for the first time.
BBC Science Correspondent Rebecca Morelle went to see one of the wave-hunting machines - Advanced Virgo, near to Pisa, in Italy.
25 Nov 2015