Moisture can get in your Philadelphia basement in a variety of ways. Leaks are probably the most obvious way, though even these can vary in how they form. Water from outside your home might start to press up against your basement walls, leaking through if the pressure becomes dense enough. These leaks can eventually expand wider and cause your basement to flood if you simply ignore them for too long. Leaks can also spring from other sources as well. If your plumbing breaks, for instance, then the water and sewage from those pipes can build up behind your walls and eventually leak into your basement as well. You might even notice a musty smell that only seems to linger around your basement because of this.
Any cracks you see around your basement will also allow moisture to seep through. This does not just include any cracks along your walls but from your floor, too. Groundwater can well up and begin to flood your basement at any given time, too, so you should not underestimate even the smaller cracks found along your basement floor. This constant flooding can also bring about other issues such as mold and mildew growth and pest infestations.