I think Uranium
Explanation:
We generally call the transuranics "man-made" elements because they are normally not found in nature. However, it has been verified that some of these elements are produced and found in nature in very small amounts. It's likely that all of them (and maybe more) exist somewhere in the universe, but the only way to get them in any useful amounts is to make them yourself.
You asked a couple questions, let's look at them separately.
First, there is nothing inherently different about transuranic elements. They do share some common characteristics. As a class, they are all radioactive. But that's also true of all elements above atomic number 82 (lead) (Pb-208 being the heaviest stable isotope known). And radioactivity is actually far more common than stability if you look at all the known nuclides. The transuranics also share the trait that they will undergo nuclear fission. We'll look at that later, also check the link for more info. But these traits are not unique to the transuranics. Uranium is also fissionable, and elements as light as atomic number 88 (radium) may undergo fission (but not very efficiently). We also should keep in mind that there are many isotopes that are much lighter than uranium which are only available by "artificial" means. So, again, there's no unique property that applies only to "man-made" isotopes or transuranic elements.
Secondly, you asked how these elements are created. The short answer is that they are produced in particle accelerators and nuclear reactors. For the long answer, let's examine the actual process of how they are made. Let's think for a moment how we would create any element - whether it be transuranic or otherwise.
Atoms are collections of neutrons and protons (with some electrons attached to complete the picture). As you probably know, changing the number of protons changes the element. So an atom with one proton (regardless of how many neutrons it has) is hydrogen, an atom with two protons is helium, etc. So if I want to create atoms with, say 10 protons, I have to figure out a way to get ten protons together along with enough neutrons to keep those protons together. It seems logical that the easiest way to do that might be to start with atoms having nine (or maybe eleven) protons, and see if I can add (or subtract) one from there. So, I need to cause a nuclear reaction, or change. Nuclear changes take lots of energy, because there is tremendous force (called, the strong force) holding nuclear particles together. This is different than chemical reactions - which only involve electrons. Electrons aren't held in atoms with nearly as much force as nucleons, so there's not as much energy needed to rearrange them. So, we have to impart energy to the nucleus. This is done by hitting it with something. Now, nuclei are very selective about what they will allow themselves to be hit by. They have a kind of "energy shield" around them. To get past this shield, other particles generally need lots of energy. So, one way to hit the nucleus is to raise the energy of some particles high enough to do that, and fling them at the nucleus - a particle accelerator. The idea is basically to shoot particles at a particular target material having properties that will result in the desired product material. Some particles work better for this than others. Very high energy photons (extremely energetic x-rays) will also do this - but you need an accelerator to produce these photons. So, particle accelerators like Jefferson Lab can and do change the nuclear structure of the materials exposed to the particle beam. This is how many of the radio-isotopes for pharmaceutical use are made. In the case of some of these rare isotopes, there is only one facility (a National Laboratory accelerator) at which the material is produced.
