Dilbit is particularly problematic in water. In the ground, bitumen is thick and heavy, more like a solid than an oily liquid. It's extracted from sandy deposits in Canada by either mining or heating the oil underground until it can be pumped to the surface. It flows through pipes with the help of chemicals that thin it out. But when exposed to the environment, those components can separate, allowing volatile chemicals to evaporate, leaving a sticky, heavy sludge that sinks in water instead of floating in a slick like other forms of oil. As it weathers, the oil gets stickier, adhering to its surroundings, and becomes more difficult to clean up over time.