It should be noted, and it is a good place to start, that our world was quite different before the flood. Many assume that the world has been very similar throughout much of its existence, yet this can be shown to be quite false. Before the flood that ravaged our world there was much more oxygen and atmospheric pressure. This has been shown in tests done by many scientists, both Evolutionist and Creationist. The following is a link that discusses a study of the Earth's past oxygen content. www.thefreelibrary.com/Amber+yields+samples+of+ancient+air.-a06122719.
The oxygen in the pre-flood environment was approximately 30%-50% more than it is today, this has been extrapolated from amber samples by scientists. The repercussions of this are that with increased oxygen content in the air, living organisms grew larger. This is evidenced by the fossil record in that many of the animals in it are virtually the same as they are at present day except for the fact that they are much larger. It has been stated by numerous scientists that the only reason insects do not grow as big as they did in the fossil record is because the atmosphere lacks enough oxygen to allow it. Our atmosphere currently contains on average, about 21% oxygen, whereas in the past, it contained about 35%. The increased oxygen would have fostered the growth of every living thing on the planet. Just as well, the organisms would have been able to live much longer. This is based on the science of living cells. Death and decay are caused by cells dieing and not being replenished, however it has been demonstrably proven that with increased oxygen levels above today's atmospheric standard that cells will replenish at a much higher rate than they normally do nowadays. There have been numerous experiments done on humans, animals and plants that show that with increased oxygen living things will grow much larger and heal much faster. This, combined with stronger and purer genetics, would have allowed organisms to grow larger and live longer and likely the genetics aided in such long lifespans as well.
In addition to a higher oxygen content, there was also twice the atmospheric pressure than we currently live under. An increased atmospheric pressure would also help living things grow much larger and live longer. An increased atmospheric pressure allows plasma to become saturated in oxygen, allowing the organism to do more while expending less energy. The increase in pressure would allow the bodies of organisms to take on oxygen much easier since it would make breathing easier. It is well established by experiments that increased atmospheric pressure, about twice as much as today's standard, will lead to an large increase in organism growth as evidenced in experiments with hyperbarric chambers at several institutions.
Another major factor in the pre-flood environment is the decreased UV radiation that made it to the Earth's surface. As with the experiments regarding increased oxygen and increased pressure, experiments have been done showing that if a living thing receives less UV radiation form the sun than normal while coupled with increased oxygen and pressure, the organism will flourish unlike anything that occurs in nature today. UV radiation is necessary, but only in much smaller doses than are received by organisms today, excess radiation of any kind is harmful to life and will cause severe problems and even death if it is absorbed in too high of doses.
How can these factors be explained? Many subscribe to the water vapor canopy theory. This states that before the flood occured, there was a layer of water vapor surrounding the planet which prevented much of the incoming ultraviolet radiation from making it to the Earth's surface like it does now. This vapor canopy would have also acted somewhat akin to a balloon, holding in much more pressure than we have today. This canopy would have held in more of the heat on our planet and caused a greenhouse affect. Higher oxygen content, higher atmospheric pressure, less UV radiation being let into the biosphere and tropical temperatures would have been the perfect environment to foster growth and biodiversity among organisms, this has been shown in experiments in which people have mimicked pre-flood conditions resulting in 40 ft tall tomato plants that yielded 15,000 tomatoes, Red bellied piranha growing from 2" to 16" long, Drosophilia fruit flies with lifespans three times longer than normal. These results do not entirely prove the canopy theory true since there are issues with it such as no one has been able to satisfactorily reproduce a water layer of the right thickness and density to explain the canopy but many still subscribe to the theory if not simply in a modified state. This is because it may be the best explanation we have so far.
The fact remains we will never know what this lost world of of planet before the Flood was like until God returns and we can ask Him and He restores the world to it's original state. The best we can do until then is try to put the pieces of the puzzle back together. We can infer from both written and fossil evidence that before the Flood the world was a much more habitable and beautiful place than it is now. Vegetation grew on nearly every part of the planet, even at the poles. Lush forests thrived around the globe and ten times the amount of life forms were alive and well. Most, if not all of the land mass was connected into one large super continent, none of the mountains we see today existed yet since they were formed by the Flood and likely the Earth's surface was much more flat, covered by plains, jungles, forests, and rivers. And until Sin came into the world, everything got along just fine.
According to Genesis, where Creationists start their views on history and origins, our world and all life forms on it were made in 6 days during what is called by many Creation Week. According to all reliable sources, we cannot trace history reliably back any further than about 6,000 years, this is why we believe that the Earth is only about 6,000 years old. After God made the world and filled it with animals, plants and the first two people He said that His Creation was "very good". There was no death and the world was beautiful and teeming with life. It was a tropical paradise. But once Adama nd Eve sinned, death came into the world and changed everything, Creation was no longer very good, it had to suffer and animals began to fight and eat others. It is because of this that God later sent the Flood to destroy life on Earth and drastically change the planet. We are still trying to learn exactly what the world was like before the Flood, but we can see not only from fossil evidence, but also from the written Word of God, that it was a much nicer place than it is now where living things flourished and grew to enormous sizes and lived to ripe old ages of nearly 1,000 years in some cases. We must accept Christ as our Savior and follow Him in order to see for ourselves the New Earth that will be like it once was before Sin. Perhaps curiosity will peek in some people enough to allow them to look into such things more deeply, leading them to recieve even more important rewards from our Heavenly Father.