A major gas leak can be caused by plant failure, or by a pipe or busting disc rupturing.

A bursting disc is a quick pressure release system that ruptures if the pressure in the cylinder rises above a certain limit. It is designed to release the contents so that the cylinder itself doesn't burst. If a bursting disc ruptures, there will be a sudden loud noise and a plume of white vapour as the gas is released. Also, the cylinder may fall over.

If there is a major gas leak, take the following steps:

First...

if you can do it without entering the cellar, turn off the gas supply.

Then ...

inform all staff and evacuate the affected area
call for assistance
do not let anyone go into the cellar - under any circumstances - until you are sure it is safe to do so (emergency service personnel may enter if using self-contained breathing apparatus)
open all outside doors and windows to ventilate the area
close all doors to passages leading to any place where carbon dioxide could accumulate
leave the cellar refrigeration switched on - the fans will help to disperse the gas
if a bursting disc has ruptured, the cylinder surface temperature will be below freezing - do not touch a frosted cylinder without wearing protective gloves.

There seems to be a problem with a gas cylinder

There seems to be a problem with a gas cylinder