As I've blogged before, groundwater works a number of ways on and within the earth to change the landscape over time. Stream bank erosion and possibly caves are the most visible and obvious examples. But groundwater can work in other ways as well.
Recently a group of geologists from Brigham Young University studied the dissolution of basalt by groundwater on the picturesque island of Oahu in the Hawaiian island chain. Yes, you heard right - Oahu is dissolving due to the movement of groundwater through its rocks. In fact, the dissolution rate via groundwater movement is reported to be faster than the erosional rate of surface materials being carried away by the streams and rivers. The highest hills of the Koolau and Waianea Mountain ranges are projected to eventually become flat plains - comparable to the Pacific Island of Midway.
The study says this isn't going to happen too soon, though, perhaps taking one and a half million years before the flattening effects start to happen noticeably. So, you don't need to cancel your vacation reservations on account of this. Besides, they also found that Oahu is at the same time being pushed upward ever so slowly by tectonic plate action, so things (elevations, anyway) may actually stay pretty much the same. But it was interesting to find out that the basalt rock of the Hawaiian Islands will literally dissolve over time. I didn't know that.