The Terminal Velocity achievement requires you to fall 30,000 feet.

How to Unlock the Terminal Velocity Achievement

There aren't any places in the Aperture Science Enrichment Center where you can fall far enough, but your Portal Gun allows you to fall indefinitely if you wish. To do this, you will need to have reached Test Chamber 11, where you gain the ability to create both blue portals and orange portals.

Find a place where you can make a portal on the ceiling that faces a portal on the floor, such as Test Chamber 15. Turn on portal funneling in the game's options, and make a portal above you and then directly below you. Point your Portal Gun straight at the ground, and you should be able to fall into the portal forever.

This achievement takes a while to be completed, even in the biggest rooms that you can find in the game. You will have to leave the game alone for a while as you continue to fall. Five minutes should be sufficient.