I assume they were doing the B4-5 extension?
IE going through the long, tiny tunnel into the much larger and deeper system (from memory it goes all the way down to the river, or at least a sump), rather than doing the shorter and easier route which lots of school groups do.
My vague memory of the sketch of the extension is that it looks very long, very low and very cramped. I can see how even a little bit of water pooling could well render it impassable... |