On March 11, 1864, the reservoir of the brand new Dale Dyke Dam was being filled for the first time. But what should’ve been a celebratory feat of engineering soon turned into a terrible disaster.

The dam broke as water poured into it, thanks to a crack in its embankment. The bursting of the dam sent a wall of water—more than 690 million gallons—crashing toward the city of Sheffield, killing more than 200 people and destroying more than 600 homes.

The flood was one of the worst peacetime disasters in the United Kingdom and, according to some, the 20th most fatal flood worldwide. Insurance claims following the disaster were the highest of any incident in the Victorian era.

The dam was rebuilt upstream, but signs of its old site still linger. A simple stone marker inscribed with the words CLOB (which stand for Centre Line Old Bank) now resides where the ill-fated structure once stood. Close to the dam are two other memorials, one erected by the local historical society and one by the British Dams Society.