Caulk doesn’t just give your shower and tub a seamless look where the surround meets at the edges. It’s also key for keeping moisture from getting behind the surround and soaking in around the tub’s sides. Even the best caulk tends to become discolored with mildew without regular replacement. Scrubbing will eventually stop removing the gray, brown, or black spots that form. Even if the caulk is still spotless, it develops cracks after three to five years, letting moisture silently seep through. If you’ve noticed water doesn’t bead up over the caulk line as it should, it’s likely leaking.