The Ancient Forest is an optional area that can be reached after you have defeated Ultimate Weapon or after you have bred a Mountain Chocobo (refer to the Chocobo Racing and Breeding side quest page for more information).
It contains a host of useful items including the following:
Apocalypse is an important weapons because it allows for Materia growth at triple the regular rate when equipped to Cloud.
Click on the link below for more information on defeating Ultimate Weapon if you have not already completed that side quest first. Defeating Ultimate Weapon is a prerequisite to completing the Ancient Forest side quests:
Land the Highwind close to Cosmo Canyon and walk through the crater. Use the screenshot below to help you find the correct location.
You can press the 
Button at any time while in the Ancient Forest in order to return to the entrance. This is helpful if you get stranded or lost somewhere but you will have to redo everything you had done to get back to where you were. By utilizing this walkthrough there should be no need to reset at any point.
Start off by crossing the upper bridge-like walkway over to the right side of the screen. There is a small item bag inside of a large red flower. Be careful not to walk too far into the center of the flower or if will eat you (causing damage) and spit you back out.
Press the 
Button repeatedly as you walk across the flower so that you pick up the item bag as soon as you can without venturing too far into the middle. The item is the 
Supershot ST weapon for Vincent.
There are three bouncing insects in this area and one bouncing frog. Press the Button near one of the insects and you will get a quick explanation about how insects and frogs work in the Ancient Forest.
Pick up the three bouncing bugs and bring them to the right side of the screen near the blue pitcher flowers that look like jugs with lids on them. The plants will eventually eat the bugs so you have to be fast at getting all three bugs into the flowers.
Use the Button to restart if any of the bugs get eaten by the flowers before you make it across. Finally there is a 
Spring Gun Clip for Red XIII on the far right side of the screen. Proceed to the right to enter the next screen.
Grab the insect at the entrance to this screen and continue to the right until you reach a large piece of a flower hanging down and the screen will prompt you to either “Jump” or “Stay”. Choose to jump up to the next level and then put the insect that you picked up into the blue flower.
Jump across the flowers to reach the other side and then grab the bouncing frog to the right of the flower.
Continue to the right until you reach two more open blue flowers and place the frog inside the blue flower on the left. Have Cloud jump onto the flower and wait until the frog bursts out of the flower. This will launch Cloud up to the red platform with the grey beehive item.
Pick up the beehive and jump off the ledge. Approach the red flower and press the Button when you are near to it to have Cloud throw the beehive into it. Now you can pick up the yellow 
Slash-All Materia sitting on the ground.
Pick up the frog again and place it in the flower on the right side this time. Have Cloud jump onto the blue flower and wait for the frog to jump out of the flower again to launch you across to the right side of the screen.
The item bag that you can see on the tree truck in the foreground cannot be obtained just yet. Continue to the right to find the next screen.
There are two bouncing insects close by which you need to throw into the blue flowers again. Remember that it is easier if you grab the furthest away insect first. Put both of the insects into the flowers and then hop across.
The last flower will launch Cloud up into the treetop area shown in the screenshot below.
Walk to the left and follow the trunk of the tree which will take you back down in the second screen to grab the item bag you saw on the tree trunk. It contains a 
Minerva Band armor. You will have to use the frog to get back to the third screen again.
Go back up to the treetops but this time head upward and follow the branch around to the left and then over to the right side of the screen. Rather than heading downwards towards the exit at the bottom of the screen follow the pathway leading upwards towards the broken branch (shown in the screen shot below).
Jump across to the far right side of the screen to pick up an item bag that contains the 
Typoon Summon Materia. Travel to the top of the branch to jump back over to the broken branch area and then take the exit out the bottom of the screen into the next area.
Grab one of the insects on the right side of the screen and throw it into the flower furthest to the right in this screen. Jump onto the flower and press back on the directional pad to have Cloud jump up to the next flower and then up onto the red platform with the beehive. Grab the beehive and toss it into the flower.
Pick up another insect and place it into the blue flower just to the left of the red flower that you threw the beehive into. Grab another insect and place it front of the opening in the tree to make a frog appear.
Now grab the other insect and place it into the blue flower to the right so you can jump across, but grab the frog and take it with you as you jump over.
Place the frog into the blue flower furthest to the right and then jump onto the blue flower. Wait for the frog to burst out and you will be launched over to the other side of the screen.
There is a treasure chest at the bottom right side of this cave that contains the Apocalypse weapon for Cloud. There is another treasure chest to the north that contains an 
Elixir. You are now done everything you can do in the Ancient Forest. Take the exit at the top of the screen to be immediately returned to the world map.
