This is the Mount Aso Crater visitors' area. It is subdivided into viewing sections and each section is subject to closure if the toxic cloud drifts that way. While we were there the wind changed direction which led to section B1 closing. It's not unusual for the entire area to be declared too dangerous at certain times, in which case the guard closes the access gate back down the road.

Even though we weren't directly inside the plume, the nearby air did get me coughing and I had to head back to the car. The internet informs me the cloud contains carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulfide, and hydrogen chloride amongst other goodies. I couldn't detect any sulphur smell myself, and I suspect my cough came from the HCl.

Unfortunately there have been fatalities from gas inhalation, and we were shown a warning sheet and questioned by the guard as we passed through the security gate.

Additional information:

• The authorities have a web page with the current access status.
• This 2008 English page outlines the warnings that should be heeded before a visitor considers viewing the crater.
• There is also a live feed of the mountain. The camera is located in the vistors' center.