Atmospheric pressure is a considerable force we rarely notice. That’s because air exerts its pressure on us equally in all directions. At sea level, one cubic metre of air weighs around 1.2kg which is roughly the same as a litre of milk. Gravity keeps us and our atmosphere firmly stuck to the Earth but air is a fluid that flows readily, allowing us to move through it with ease. At low speeds it provides few clues of its presence to our senses. Only when we pace through it at speed, or when huge bodies of air get moving as wind do we really appreciate air’s very significant bulk.
The air inside the inverted glass is at atmospheric pressure. When you submerge the glass, the air inside exerts enough pressure on the water to prevents it from entering and wetting the tissue.
Diving bells used in deep sea diving rely on exactly the same principle to transport divers beneath the sea. Modern diving chambers are more sophisticated because they usually double as decompression chambers, but the same principle allows divers to enter and exit the open chamber while submerged.