Due to the way the map was excavated a pocket of CO2 was formed in the small alcove next to the dig instruction in the picture below. As the water flowed over the pocket it drained and moved into the square shown. At this point the water drained through the gas pocket and compressed until all the water above had drained and the gas was able to interact with the gas above. In the absence of the gas pocket the water then expanded back to its original density.
create a airpocket, remove a wall allowing greater water pressure to fill over it.