The Abandoned Mansion is on Cinnabar Island in Kanto. It is home to many Poison and Fire type Pokemon. It also houses a few normal types as well, mainly Ditto and Raticate, you may also get Growlithe if you have Pokémon Fire Red version The Abandoned Mansion is also full of trainers. There are Mewtwo statues here because of the Mewtwo Journal in the higher level explaining that the Abandoned Mansion was the cloning facility of Mew at one point.
The main purpose of the Abandoned Mansion is to get the key that unlocks the Gym. Blaine is the leader of this Gym. It's recomended you take a Escape Rope, because it is possible to get stuck in there, at least in Pokémon Blue, and Pokémon Red. There are a few good items in there, such as Rare Candy and TM22, Solarbeam. As of the many items you can find in here, there's also a journal that is said to be the jornal of the researcher who cloned Mewtwo and he used a fossil of Mew to do it. However the expirement went wrong and the researcher has a warning in the journal not to clone a legendary pokemon. As of the first movie of Pokémon, the expirement showed to be a total failure and Mewtwo ended up making a total destruction of places it went to. In the end Mew ended up stopping Mewtwo of it's wrath and saving everybody at the lighthouse that Mewtwo was at. It is said that the Abandoned Mansion is the cloning facility of Mewtwo. There are many switches here as well that make it tricky to move around the place with, and you could end up starting at the entrace again if your not careful. Try to think a head of time before turning a switch because it could really help you out. The two types of trainers in there are Scientists and Robbers (Occasionally there are some kids you might find in there) and you'll find a lot of the trainers on the lower level near the key to the Gym. the main items here are the Mewtwo journal (Found on the highest level of the mansion) and the Key to the Gym (Found on the lowest level or the basement of the mansion). The Abandoned Mansion is a must see place whether you want to find out a little bit of info on Mewtwo or continue your gym battles by finding the key to the gym or maybe train and find certain trainers throughout the building and battle them.
In Generation II and Generation IV the mansion is burned down due to a valcano accident on Cinnabar Island so you may only get to explore the mansion in Generation I games (includes Pokémon Red and Pokémon Blue versions) and Generation III games (includes Pokémon Fire Red and Pokémon Leaf Green versions). However you may still explore Cinnabar Island in Generation II and Generation IV games but with out the mansion.