The part of the sentence 'was thought impossible' is correct and usable in written English.
You can use it to describe an event or situation that was believed to happen but then it happened or it became possible. For example: Ten years ago, a cure for this debilitating disease was thought impossible, but now it has been discovered.
Until this piece of work was done, it was thought impossible, so no one has looked.
Today the NHS has achieved what was thought impossible in the late 1990s – more than 90 per cent of patients are treated within 18 weeks.
Regular but nonrepeating patterns, defined by precise rules, have been known in mathematics since antiquity, and medieval Islamic artists made decorative, nonrepeating tile mosaics, but the phenomenon was thought impossible in the packing of atoms.
Dr Pawel Tabakow, consultant neurosurgeon at Wroclaw University Hospital, who led the Polish research team, said: "It's amazing to see how regeneration of the spinal cord, something that was thought impossible for many years, is becoming a reality".
Furthermore, the intersection of physics, computer science, chemistry, and biology has driven the development of technologies that make it possible to consider what was thought impossible just a short time ago.
Today, NCI officials outlined a plan to bring together the agency's contract lab in Frederick, Maryland, and outside researchers to find ways to block a mutated protein that drives growth in one-third of all cancers but was thought impossible to "drug" until now.
Getting a mere number-cruncher to do so had long been thought impossible.
They managed to do what had previously been thought impossible: they probed reality without disturbing it.
Forty-nine yeago ago this Tuesday, Britain's Roger Bannister achieved what had once been thought impossible.
Until then, Brazil's savannah with its acid, nutrient-poor soil had been thought impossible to cultivate.
In 1961, at Tottenham, he won the league and cup double which had until then been thought impossible.
