On a calm day when Lake Erie is like glass, visibility can reach depths not imagined in the early 1970s. Unchecked pollution at that time limited water clarity, which research shows was at best approximately three feet. After a decade of reduced phosphorus inputs, clarity improved to six to 10 feet in the 1980s. When zebra mussels dramatically expanded water clarity to 10 to 17 feet in the early 1990s, many water enthusiasts hailed their invasion as a positive outcome. However, it is crystal clear that negative impacts caused by these voracious filter feeders since the far outweigh the benefit of seeing at deeper distances.