Galactic History 80: A Return from Exile

As of patch 1.2, you can no longer get this datacron using any other methods described previously. The method has been completely altered, ie there is no invisible platform any more.

1. Enter Port Pevaria (entrance at 3280, -3095). Imp players will have to go through a Pub quest hub, so be prepared.
2. Go to 3511, -3204. There is a Jedi Protector (Level 49 Elite) pacing right at this spot.
3. Zoom out and move you camera angle so you have a birds-eye view and you will see a small silver pipe that sticks out past the platform you are standing on. You have to jump onto this pipe, however there is little room for error. The easiest method is to turn side on, then jump and tap strafe while mid-air. Rinse and repeat.
4. Lastly, simply turn around, jump onto the platform and run across it to the datacron.
(5. Note that /stuck did not work for me so you will have to suicide.)

The information submitted by Kallisti is incorrect. It's ok for Datacron 78, but not for 80.

Here's a link to a video explaining how to get #80 -> http://www.youtube.com/watch?v=dwBdjsarjAE

How to retrieve this Datacron:

This Datacron is located in the Government District of Corellia far in the northeastern section in the Republic Encampment of Port Pevaria (Screenshot 1). For Imperials getting this Datacron kinda sucks but for Republic it's easy as pie. What you need to do is enter Port Pevaria and head far northeast to where the zone ends and there is nothing but water below (Screenshot 2). Here you will see a set of brown pipes just off the edge of the catwalk that you need to jump down to (Screenshot 3).

Be careful not to hit your head when jumping to these pipes. If you want, it's actually easier to kinda squeeze under the platform above when you're standing on the railing and just drop down to the pipes. I fell a few times before figuring this out. Once you make it to the pipes head south underneath the catwalk. Helpful Tip: You can hug the wall without falling between the pipes and the wall.
Follow the pipes south until you reach where I am in (Screenshots 4 & 5). The Datacron is just ahead sitting on an invisible platform. From these Screenshots you will want to fall down onto the metal pipe I am looking at in (Screenshot 4) and run over to the Datacron. You can fall down right next to the Datacron here onto the invisible platform. To get out from here you'll need to either kill yourself or type /stuck.

To see the screenshots for this guide you'll have to use my website as I can't post them here:

http://almarsguides.com/swtor/datacron/corellia/presence.cfm

80 - [Presence +4] X: 3143, Y: -1793 The second Datacron is also placed in the Government District. To achieve it ride the piston to the top and blow the bomb. The start point is at 3118, -1843.

http://www.proprietary-guild.com/forum/m/2427303/viewthread/1954157-proprietarys-comprehensive-datacron-database