Police in Ebbw Vale are appealing for information after thousands of pounds of damaged was caused.
The incident happened on Saturday 11th November and the following day.
Officers believe large 4x4 vehicles were driven along the dam and reservoir area close to Llangynidr Road.
To do so, the drivers have smashed through a secure gate and caused damage estimated to be around £25, 000.
These vehicles would have been visible from the A465 Heads of the Valley Road.
If you saw anything that weekend or if you know who was responsible, please call 101.
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