This was was considered a 'safe cave' by the tour companies. The areas that the tourists were supposed to go into were non-silting and the dangerous passage was clearly blocked off. Until one day it wasn't blocked off and the guide turned into it.
To be technically correct, it was considered a safe
cavern. There are no caves that are considered safe for untrained divers.
In this case, the guide was untrained and did not know that the area he entered was not safe. When he made the mistake and entered, though, he should have known that it was not a good move as soon as he looked in. But he went in. After that, everyone of the people following him should have known as soon as they looked in that it was not safe. But none of them knew that. Why? Because no one had trained them on the difference between a safe cavern or swim through and an unsafe cave. Thousands of divers make perfectly safe transits through short swim throughs every day around the world, but they don't know where to draw the line. Why is one safe and the other unsafe?
If you enter the side passage ways in the upper area of the Spiegel Grove, you can pause in the door and look into the room. You see a doorway to another room, but you also see a big square hole in the hull through which you can exit easily to open water. So you have the exit you are currently coming through and another spacious one about 15 feet away. OK, then. You go into the room and check the next one. Same thing. Two easy exits in plain view. No real silt. No entanglements. If you look into another passage and see darkness, entanglements, silt, and no other exit, you should immediately think that you need special training to go there and stay out.
In the Italian situation mentioned above, everyone who went in there should have immediately seen darkness, no exit, and silt. That should have said
stay out! to them, with or without a sign.
But up until recently, there was no training of that kind available. Now there is at least a simple academic class called
Understanding Overhead Environments that can illustrate the signs that help you make the decisions as to whether or not you have the ability to enter the environment you are considering. Take a course like that, and you can hopefully make better decisions.