Officially, only 1 casualty has ever been reported.
Most people that go under have maps, or are guided by poeple with maps.
the main real danger is that the underground roofs are not so stable in some areas. That's also the reason why the underground is closed to the public.
About tours, the term is maybe a bit over, but indeed some cataphiles occasionally "initiate" rookies (like me

) to the beauty of the Underground.
In some districts, circulation is difficult for unprepared people: water up to butt, small holes in walls to go from area A to area B (chatières), low roofs in corridors, and accesses through forbidden areas (like a tunnel in a former train network called the 'PC', near "Alesia" neighborhood).
In other districts, the travel underground is far more comfortable, with large and dry corridors, huge and astonishing rooms. But often also these areas are accessed through wells that can be as "high" as 50 meters. Going under can seem easy (if not subject of giddiness (sp?)), but going out is quite qa long and harassing rise, not even speaking of raising the plate once at the street level (my friend used to do it by kicking it with his legs, head down, only harnessed by a cable...).