The amount of time a diver can spend underwater is not static. Depth plays a major part in determining bottom time, as a result of two factors: Air consumption and nitrogen absorption.

The first factor is fairly straightforward. The deeper you are underwater, the more air you consume with every breath. This is a result of water pressure. Deeper underwater = higher water pressure = decreased volume. The only thing that remains constant is the size of your lungs. Thus, the deeper you go, the more air is compacted into every breath. Greater depth means greater air consumption.

The second factor, nitrogen absorption, is also direcly related to depth. Common, everyday air is mostly nitrogen, so when you breath pressurized air you absorb a great deal of nitrogen into your bloodstream. Why is this a problem, you ask? Well, if you absorb nitrogen at depth, and then go to the surface, the nitrogen in your bloodstream expands and makes bubbles. That's a really really Bad Thing. Those bubbles can cause an embolism and kill you dead. So, you can only spend so much time on the bottom before you've absorbed too much nitrogen and have to surface before it reaches certain levels. Sometimes if you've been at depth for too long you'll need to stop for a bit partway to the surface and let some nitrogen bleed off. Depending on how deep you were for how long, you might need multiple stops.