In 17/08/2022, all the fish in the Oder River, a river that starts in Poland and flows into Germany, were found dead. The cause of death was high levels of mercury in the water.
https://www.theguardian.com/world/2022/aug/17/poland-pulls-100-tonnes-of-dead-fish-from-oder-river-after-mystery-mass-die-off

The reason there was mercury in the water was because a polish paper mill that has been dumping mercury and lead into the river for weeks, because the local sewage system refused to take the waste from the mill as they couldn't handle it. After some lobbying with the polish Parliament, the mill received a 40 days long permission to dump its sewage into the Oder.
In Poland, the entire deal was brushed under the rug. It didn't receive international attention until germans took notice of the dead fish on their side of the Oder and decided to do some tests, which have identified lethal amounts of mercury and lead in the water. The polish Parliament denied everything, claiming that they've done a test themselves and found only a high concentration of salt.
Mercury is a neurotoxin. It is also a fucking element, it does not degrade or breakdown, it will persist in nature and keep killing, yet german authorities, in fear of polish retaliation, claim that the poisoning will be fixed in 2-3 years time. The entire river has been irreversibly poisoned, and its ecosystem completely demolished. It'll take decades, if not hundreds of years for the Oder to recover, if it'll ever recover at all.
But this incident isn't the only time Poland poisoned the Oder River. Pics related.