I've just come across this thread and am pleased to see so many sensible opinions on the subject. Personaly, I spend far more time underground alone than with others, attracting a mostly critical response.
While I fully accept that were an accident to happen, the consequenses would be more serious, I also think that the chances of an accident happening are far less. I know what I am capable of and I know I can trust myself; consequently I feel far safer by myself.
Safety asside, I also find being underground alone really relaxing, you just get on with what you are doing and can completely forget about anything and everythnig that's going on above ground. You can never quite get that feeling on a group trip.
I was thinking of oil drums used as entrances or shaft liners - as in, say, Kingsdale Master Cave.
You'll be pleased to know the old oil drum was replaced a few years ago with a nice plastic tube, it seems much more sturdy, though there are still plenty of oil drum digs out there 😉
The distance between stupidity and genius is measured only by success.