1900 AD is the time era when Lavos arose and rained destruction on the world.
There are only 2 things to do in this time era. Either run away or fight Lavos' shell.
If you choose to fight Lavos, you will either be launched into a battle against Lavos' shell, which takes on the form of several bosses previously faced if you took the bucket or you will use the Epoch to break through the shell, destroying the ship. If you manage to defeat all the bosses, you will get the option to either go on and fight the final boss or run. If you run you do not have to refight the shell.