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Nature
Flat
Landscapes
natural, volcano, Erupting
Firebrick
512x512
nature, danger, Disaster
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Mixed
Krispicons- Modern Flat Icons Freebie
Attention, danger, Caution
111x111
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Photography & graphic design
3D
32x32 Free Design Icons
danger
32x32
Network & communications
Pixel
16x16 Free Application Icons
prohibition, danger, reject
16x16