Water is heavy by itself and can absorb twice its volume of dissolved sugar. According to King Arthur Flour's website, ¼ cup of molasses weighs 3 ounces; cookbooks tell us that the same amount of water weighs about 2 ounces. That couple of million of gallons of sugar syrup oozed along at 35 miles an hour, breaking and crushing everything in its path. People and animals drowned in it. It wouldn't be possible to swim in any such substance, and at some depth, running through it would become impossible. Getting out of it would be rather like escaping the Tar Baby.
With a clear, open road ahead and a good long start you might outrun it. How many roads in the North End are open and runnable now? How many were then? If you saw this dark-brown mass surging toward you, how long would it take to figure out what it was and that it would be appropriate to run like hell in the opposite direction?
Twenty-one people died in that catastrophe. Imagine going through life explaining that your husband or mother died in a molasses flood -- it sounds like a joke, horrible as it must have been.
It is said that in hot, wet weather some cellars in the North End still smell of molasses, the remnant of that dark tide.
If you don't believe the above, you may consult: