Police were forced to evacuate a cruise ship full of tourists this week after it ran aground on a dried-up Danube river. The Danube River, which runs through 10 countries from southwestern Germany to the Black Sea, has receded to never-before-seen lows along much of its course, exposing rocks, sand bars and old shipwrecks that have rarely been seen before. In Bulgaria, where the Danube forms a border with Romania, police on Tuesday evacuated 186 tourists from a cruise ship that had run aground near the port city of Vidin. Hidden treasures and dangersThe exposed river bed can also pose dangers. Read more: How river cruise lines are dealing with low water levels