Complex sentence: Some of the oil from the ship was dumped in a quarry on the Chouet headland, on Guernsey in the Channel Islands, and is still there today, however efforts to rid the island of the oil continue, with some success.
Split into simpler sentences: Some of the oil from the ship was dumped in a quarry on the Chouet headland, on Guernsey in the Channel Islands, and is still there today. However, efforts to rid the island of the oil continue with some success.