The Bloons were first created by a malfunctioning machine. A carnival worker named George, frustrated with having to inflate hundreds of tiny balloons for the dart-throwing game every single day, decided to build a machine that would create and inflate large amounts of balloons on its own. By 2004, it had become widely used at fairs and carnivals all around the state.
In May 2004, George was attempting to take the machine to a party, where hundreds of balloons were due to be dropped from the ceiling by a net. On the way to the party, George was in a car accident which sent his car over the edge of a small cliff and into a nearby lake. He drowned inside the car; however, the machine was kept safe inside a large, watertight trailer attached to the back of the vehicle.
Six months later, the Environmental Protection Agency removed hundreds of discarded barrels of nuclear waste from the lake, accidentally taking the trailer with them. The trailer was taken to a proper nuclear waste dump and abandoned there for two years. It was not until March 5th, 2007 that the mutated, irradiated balloons, nowadays called Bloons, began to terrorize the civilians and endanger hundreds of lives. They had to be destroyed from a distance using darts and other weapons to keep them from popping themselves on monkeys and injuring them.
The location of the original Bloon Machine is unknown and unreachable to the Monkey Military, as nobody has been able to get anywhere near it since it started creating Bloons. The Legion has upgraded the original machine many times over the years, and additional machines have also been built, at the Bloons' command.
Many normal Bloons are not intelligent or sapient. The first Bloons were like ants, following each other mindlessly, knowing only violence. However, over the years, consistent radiation plus help from the Legion has resulted in a large majority of Bloons being sentient in one way or another. Long ago, they attacked without reason or knowledge, but nowadays, they are simply stuck in their roles, unable to imagine a world where they don't have to fight the monkeys, where being popped too soon isn't a fact of life.
A default MOAB is more intelligent than most regular Bloons, the vast majority being comparable to a 10-year-old child in terms of intelligence and reasoning skills. Before other Bloons were created, MOABs were the leaders of the Bloons. A BFB is slightly more intelligent than a MOAB, a ZOMG has the intelligence and reasoning of a 10-year-old, and a DDT or BAD has the intelligence of a 15-year-old. A DDT is a highly intelligent targeted attacker, not too unlike a sniper. The sentient ones among the Bloons are simply too angry at what happened to their predecessors, to ever forgive the monkeys; and those who can forgive them, are sent out to fight anyways.
The Boss Bloons are comparable in intelligence to an above-average adult monkey. They can outsmart many of the heroes, and are fully sentient and sapient. They would definitely be able to comprehend and uphold a truce if one was offered and they wanted it. But the Boss Bloons have no interest in stopping their attack, and are angry at the monkeys for ever attacking any Bloons, even if the Bloons were mindless objects of destruction when the war started.
(However, they fail to acknowledge that the Bloons, though underdeveloped and non-sentient when the war started, attacked first, and that the monkeys were always just defending themselves. What were we supposed to do? Come on, Bloonarius, be reasonable here...)
The high levels of radiation used to make Bloon machines produce living Bloons instead of inanimate balloons also cause occasional variations and mutations while Bloons are inflated. For example, you might see a Purple Bloon with a Lead shell around its middle, or a Pink Bloon with random black stripes, or a ZOMG with only one propeller, or a very tiny MOAB that had to be sewn back together by Legion monkeys because her own ceramics broke her apart. But these Bloons are sent off to do other jobs, never being allowed to fight for fear of making the Bloons seem weak or imperfect. However, one might say that this enforced perfectness is exactly what has allowed the monkeys to see the Bloons as a faceless enemy rather than their own species...
Bloon reproduction works somewhat similarly to cell mitosis - a healthy, safe Bloon, given enough time, will produce another one. If the outer layer of a fully-grown Bloon is not popped (or, for Leads & Ceramics, has not sustained significant damage) for six months, the process will begin.
Week 1: A thicker, tougher spot will gradually form on its underside, about 3 inches in diameter.
Week 2: The spot's edges start to shrink inwards, forming a small and wrinkly bump.
Week 3: The edges fully meet, forming a seal between the developing Bloon and its parent. The rubber forms a connecting structure called a stem, which is a solid cylinder about one inch long and half an inch thick.
Week 4: A tiny hole forms in the side of the stem, about halfway down. The upper stem segment then hollows out, forming an L-shaped tunnel allowing airflow into the shell of the developing Bloon.
Week 5: The shell forcibly expands, sucking in air until it reaches a diameter of 6 inches.
Week 6: The tunnel closes up, leaving a weakened spot halfway down the stem. The child Bloon detaches from its parent. Over the next two weeks, the remainder of the stem is re-absorbed into the parent's rubber.
The newborn Bloon is weak, vulnerable, and slow, but is sentient and can communicate. Its only layer is the same as its parent's outer layer. Over the next six months to one year, it grows from the outside in, layer by layer. For example, a newborn Pink bloon only has its outer shell at first, but then develops a Yellow layer, then a Green layer, then a Blue layer, then a Red one. From one to three months old, it automatically takes in more air and grows in size rapidly. By three months old, it is already close to its full size (depending on what type of Bloon it is - the more layers it has to grow, the more time it will take). Now that it has more air inside it, it is more mobile, and can move at the same speed as a fully-grown Bloon of the same type. At the age of four months, it can voluntarily take in or let out air to adapt to changes in air pressure at the same rate as adults. Once all the layers are fully developed, the Bloon is fully grown.
A Bloon born through splitting, as opposed to having been inflated by a machine, will have no knot, but the stem is never re-absorbed by the child, so it stays in place of the knot.
The healthiest and most comfortable air pressure for a Bloon based on the amount of air inside it is called its adjusted pressure. At its adjusted pressure, its innermost layer will be inflated to a diameter of 18 inches. They can change their adjusted pressure by slightly loosening their knot and taking air in or letting it out, but this takes a long time to do safely and is rather difficult for the Bloon. Without loosening the knot enough to undo it, they can lower their adjusted pressure by 25 kPa per hour or increase it by 5 kPa per hour. However, they need to constantly focus on doing this the whole time. (For Bloons born through splitting, the tunnel in their stem can open slightly for this same purpose.)
Inflation level is the difference between the adjusted pressure and the ambient pressure. A negative inflation level indicates underinflation, and a positive inflation level indicates overinflation. For example, a Bloon with an inflation level of +40 kPa is adjusted to a pressure 40 kPa over the ambient one, and is now dangerously overinflated. This would happen if it was suddenly taken from 100 kPa (360 feet above sea level) to 60 kPa (13,800 feet above sea level).
When the ambient pressure is much higher than the adjusted pressure, the Bloon will appear small and have a hard time moving, but will not pop or be in pain. If their inflation level is under -50 kPa, the Bloon will have visible wrinkles, and be stuck on the ground until they can take in enough air to float again. Thankfully, they are harder to pop in this state, as their layers aren't stretched as much and are now thicker.
When the ambient pressure is much lower than the adjusted pressure, the Bloon will start to overinflate. This is uncomfortable at first, and then becomes very painful. If they are popped in this state, or explode next to something to damage it, they will burst with much more force, and do more damage to whatever's nearby. They become fragile and volatile at +20 kPa, and will pop immediately at +50 kPa.
Bloons who are not able to fight, or were forbidden because of their mutations, shelter in Legion bases, performing other tasks for the Legion or simply hiding away from non-Legion monkeys, who will pop them on sight or run away and call for help. Some hide underground instead.
If the war were ever to end, the Bloons would live in tall, old-growth redwood forests, building civilizations in the treetops. They would also live in floating cities among the clouds, or in the mountains.