WELCOME to Ballarat - a once-thriving goldfields town where visible signs of its rich history still abound today.And sometimes where you least expect to find them.Our story yesterday about an old mine shaft opening up in a reserve in Dyte Parade, Ballarat East, has prompted calls from others with similar tales, some recent and some not so.It seems recent rains, the likes of which we haven't seen in years, may be contributing to the sinking and subsequent re-opening of old mine shafts around the town.More than a century ago, Ballarat's landscape (far more sparse than it is now) was dotted with mine shafts. As they reached the end of their life, they were filled in, generally with whatever was available at the time.Over the years, many of the shafts have been built over or developed into recreation reserves or parklands.With the passage of time, the grounds settle, the fill shifts and, eventually, someone wakes up to find a hole in the backyard that wasn't there before.Such are the hazards of living in a city built on gold.Fortunately, these collapses have been sporadic and without injury to human life in recent times.But the potential for hazard is there.More rainfall overnight increases the potential for more shafts to open up.That's not to say the city will disappear into a sinkhole. The re-appearance of a long-forgotten shaft happens so infrequently that it still has novelty value - hence this newspaper's interest when it does occur. In a place like Ballarat, though, there is always the chance of encountering a disused mine shaft, particularly when exploring the beautiful bushlands around the city.The re-appearance of a couple of shafts this week will be a curiosity for some people, but it is also a timely reminder of the need to take care when we are traversing old mining areas.