Just as another explanation, if materials can get out, it is also simple for materials to get in. In the case of flooding, it is not unlikely that water mixed with dirt getting in and drying. This isn't inconsistent with the material that they pulled out. It will likely be impossible to know which the real answer is, but I'm not holding my breath. The joke seems like the easier explanation. Let's make sure that what we want to believe doesn't prevent us from accepting reasonable alternatives (this applies to both of us).