I first happened upon this marvel of engineering on this recent CNN Travel video story [cnn.com]. Digging around the internet, I then found this late April story on CNN.
London's new see-through Sky Pool is first of its kind [cnn.com]:
The Sky Pool is a 82-foot (25-meter) transparent swimming pool stretched between the 10th stories of two residential skyscrapers in southwest London's Nine Elms neighborhood -- and it's only open to the apartment complex's lucky residents[*].
[...] The pool was put through extensive strength testing at the Reynolds factory [in Colorado] before making its journey to the UK by road and sea. It was then lifted into place by a 750-tonne mobile crane, supported by a 50-tonne crane.
[...] "After a series of technical drawings and behavioral analyses, the dimensions of the pool were decided," says the Embassy Gardens website."
With sides 200 millimeters [(7.9 inches)] thick and 3.2 meters [(10.5 feet)] deep, and with a bottom 300 millimeters [(11.8 inches)] thick, the 50-tonne acrylic pool will span the 14 meters [46 feet] between the buildings, with steps and filtrations systems sitting either end, and five modes of lighting to add to the feeling of magic."
[...] "Once you swim off, you can look right down. It will be like flying," says Brian Eckersley, director of Eckersley O'Callaghan.
[*] a two-bedroom unit starts at just over £1 million ($1.4 million).
Entry on Wikipedia [wikipedia.org].