An army bomb squad blew up a container filled with explosive chemicals that was found at a girls' school.
The experts carried out the controlled explosion on a beach near Santa Sabina secondary school in Sutton.
Teachers called the bomb squad after discovering dinitro-phenyl-hydrazine during an audit of their laboratory. The chemical is safe when wet, but highly explosive if dry.
Soldiers isolated the chemical, which is better known as Brady's reagent, and dealt with it after classes finished.
A defence forces spokesman told the Herald: "The substance can become unstable as it decomposes and dries out, and then becomes sensitive to shock such as being dropped or banged.
"We handle about two of these a month. The usual procedure is to wait until the school is empty and take the chemical compound to an open green area, where a hole is dug.
"It is then placed inside with an explosive charge and the hole is covered with sandbags before the charge is detonated.
"In this case, the tide was out and it was convenient to take the chemical there to dispose of it."
The scene was declared safe a short time later and the squad returned to their base.