[JimSB]

When you start out, the limited oxygen limits your cave exploration. However, when you get into the mid to late game, you will have acquired the following:
A good spacesuit with lots of hit points, 120+ oxygen, good damage reduction and visibility

a) Field Medic 2 and/or self-sealing suits to heal damage - absolutely essential, as you can shrug off minor damage.
b) A good weapon with range 4+ and adjacent damage
c) Hand Scanners are good
d) Spelunker skill is good too, as it allows you to locate cave exits/entrances

If you have the above, you can do a deep cave run, which can give you literally tons of equipment, crafting parts, trade goods, and the odd Firaxian clue as well.

Not all caves go deep, but some go VERY deep. I got to level 95 in one in my last game and could have gone further.

As you go, pick up oxygen and keep going. In deep caves you will usually be able to pick up enough so that you will never run out. In one of these, I had over 5000 Oxygen at one point.

Some things to note:
Your team can carry an infinite amount of resources, oxygen, and crafting parts. However, you can only carry one piece of equipment. Fortunately, you don't have to retrace your entire path when you pick up something nice. Instead you can use a teleport (on the red-eye raider levels) or the the cave exit (other levels) and you can go directly to the surface and return to the shuttle. At that point DO NOT take off again, or you will lose your accumulated oxygen. Instead, return to the cave where you will be given the option to go to the deepest level you have reached so you can continue where you left off.

If you are carrying a good piece of equipment, and find another be aware that if you leave that cave level, the NEW piece of equipment will not be there when you return. Instead, drop what you are carrying (Abandon cargo), pick up the new equipment and return to the shuttle as above. When you return to the cave, what you dropped will be there.

Beware of disease carrying critters - with an well-equipped party, these are about all that can stop you.

Enjoy