Rafael told me his memory of the terrible accident which happened in 1971. A bus, travelling from Barcelona to Badajóz fell off the bridge and was swallowed by the river. 45 people were rescued and 9 were lost, including 5 children. Apparently thousands of people went to help.
I read that the well or chasm is man-made, probably from first-century excavations to provide stone for the bridge. It was eventually brought from further away because what was extracted wasn't good enough. The chasm is up to 18 metres deep.