[DrainPondDesign]

A tragedy yes but unfortunately this is what NORMALLY happens with this sort of thing.

A person goes down the hole & collapses; the observer sees no hazard & thinks it may be heart attack & goes down to rescue them. Now you have two victims & more may follow for the same reason.

It doesn’t have to be explosive or even smell bad; it can just be the lack of oxygen which without instrumentation you can not detect.

People have died inspecting drained potable water tanks. Drinking water, what could hurt you? The rusting steel uses up the oxygen & you asphyxiate.

The way to avoid this kind of tragedy is to know before you go but when it's a loved one in trouble even that may not be enough. I'm not sure I could have resisted going in to help either.