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More than 1,000 toads have puffed up and exploded in a Hamburg pond in recent weeks, and German scientists have no explanation for what's causing the combustion.

Both the pond's water and body parts of the toads have been tested, but scientists have been unable to find a bacteria or virus that would cause the toads to swell up and pop, said Janne Kloepper, of the Hamburg-based Institute for Hygiene and the Environment.

“It's absolutely strange,” she said. “We have a really unique story here in Hamburg. This phenomenon really doesn't seem to have appeared anywhere before.”

The toads at a pond in the upscale neighbourhood of Altona have been blowing up since the beginning of the month, filling up like balloons until their stomachs suddenly burst.

“It looks like a scene from a science-fiction movie,” Werner Schmolnik, the head of a local environment group, told the Hamburger Abendblatt daily. “The bloated animals suffer for several minutes before they finally die.”

Biologists have come up with several theories, but Ms. Kloepper said that most have been ruled out.

The pond's water quality is no better or worse than other bodies of water in Hamburg, she said, and the toads did not appear to have a disease. A laboratory in Berlin has ruled out the possibility that a fungus has made its way from South America, she said.

While tests will continue, Ms. Kloepper said, city residents are warned to stay away from the pond.