A couple years ago, I was diving near Puerto Rico, in the general vicinity of a couple Humpback Whales - a mother and calf. They weren't showing themselves, but their voices were in my ears from the moment I crossed the barrier between air and ocean. It was haunting, wonderful, and a little unsettling, like intercepting the transmissions of an alien species broadcast across the cosmos. You might not think that merely overhearing the grunts and whistles of another creature, perhaps over a mile away, would be quite so profound, but it perfectly underscored the reality that as a diver, my time and presence under the surface was transitory, and that I was totally unequipped to survive in this vast environment. I was trespassing in an ecosystem of astonishing diversity and vitality - one that rivals our own, yet which is often considered barren and featureless. It's one thing to understand this intellectually, but hearing them sing over the blue distances was almost a religious experience, a moment in the awe-inspiring presence of great and unknowable entities.
Last week I had the opportunity to go see some whales, not from under water (which would probably leave me with my mind permanently blown) but from the safety and confines of a boat off Cape Cod. For whatever reason, what was most compelling about being in their presence was hearing - and feeling - them breathe. There is an explosive gasp of exhalation as they surface, accompanied by a plume of vaporized seawater, and if they're close enough, you can see the muscular movement of the blowhole and hear the great, deep intake as they fill their lungs for another dive.
This got me curious about the blowhole. There is a great deal of diversity among mammals, but it seemed like rerouting the cetacean airway was one of the more extreme and interesting adaptations in mammalian evolution.
It turns out that there's a clear evolutionary path visible in the fossil record. As illustrated in this article from the terrific site Understanding Evolution, the ancestors of modern whales had the nostrils-forward morphology that's typical of most mammals. Over time, their external nares migrated progressively to the top of their muzzle, backwards and higher on the head. This conferred an obvious advantage in streamlining and in keeping the head and sensory apparatus under the surface while still allowing respiration.
It also means that the biology of their airway differs from ours in some surprising ways. The airway is completely separate from the esophagus, meaning that whales can't breathe through their mouths. (I'm guessing that this is to prevent the aspiration of water or food into their lungs.) Another result of this separation of function is that whales can't use their mouths to make sounds. In humans, the intricate musculature of the mouth, powered by air blowing over the vocal cords, allows us to produce a wide range of expressive noises: language.
Yet sound is even more important to the cetaceans than it is to us terrestrials: whales probably gather far more information about their surroundings via sound than they do by sight. Whale vocalizations may be audible several thousand miles under the right conditions.
The mechanism by which baleen whales vocalize (without exhaling any air) isn't yet fully understood. I don't mean to go all Star Trek IV on you, but from a lay person's perspective, it doesn't appear we fully understand why they communicate in such complexity or what information is encoded in their vocalizations. Are they merely navigating the wide deeps, or facilitating sexual selection, or confirming identities and group identity? Or is this the language of minds that have evolved in a harsh planetary environment we can barely imagine, and into which we can only briefly and tangentially trespass?
P.S. Wikipedia's entry on whale vocalization contains some good info and links to much more.