Warp

Here's a bunch of helpful info:

How to return/backtrack to Laboratory (2nd floor) West section for missed collectibles (two options by going out of bounds):
1. https://vimeo.com/37628577 (this is on floor 1 where you first encounter the alien mines leading to the drop to the alien boss fight)
2. https://gamefaqs.gamespot.com/boards/619744-warp/62132327 (last post)

Once you backtrack there it also becomes possible to lift the bridge leading to the East side of the Laboratory if you missed collectibles in that section (you can also reach that East section by taking a very long way around through several floors and lifts/elevators). Make sure you activate the transport tube on the west side first and then swap an Energy Barrel from the west to the glowing pink slot of the energy generator next on the right side of the big pit (nothing will happen) and then remove the barrel from the pink slot (bridge now rises).

Every other location in the game can be backtracked by going through various floors and lifts/elevators (before taking the final transport tube to the submarine, obviously).

Bonus! Here are all the lifts/elevators destinations:

Floor 1
- north-west elevator goes to floor 3 north-east
- east-most 'elevator' (one-way drop) goes to floor 2 south
- second east elevator goes to floor 2 south

Floor 2
- south elevator goes to floor 1 east
- east elevator goes to floor 4 south
- west elevator goes to floor 4 east

Floor 3
- south-west elevator goes to floor 4 north
- north-east elevator goes to floor 1 north-west

Floor 4
- south elevator goes to floor 2 east
- north elevator goes to floor 3 south-west
- east elevator goes to floor 2 west