1. Here's a fun puzzle. You need to stack several (here I have four) cinder blocks so that the board becomes a ramp to the ledge beyond. Neat!

2. Next up is this unfriendly chopper. Note the medkit on the left.

3. Another medkit is here on the right.

4. Head left to reach this ledge. Kill the barnacle to safely grab the medkit beneath his tongue. Move the barrels away from the blue door and enter the building.

5. A ladder on the right leads to the lambda below, where you'll find two medkits. Return and continue left.

6. Ammo is in this crate, and an energy cell lies to the right. Swim left to find more supplies before returning here and proceeding to the right.

7. I had better luck with the barrel-in-a-barnacle trick this time. Shoot the flammable barrel to see how many barnacles you can take out at once. Continue to the left.

8. Same deal. Again, the way out is to the left. This time a medkit is also to the left.

9. Ascend some stairs, pass through a door then keep left to find these supplies. Continue through the tunnel ahead to load the next map segment.

10. Explode this barrel to clear the path.

11. Clear these boards with your crowbar.

12. This nice man was about to open the door when these Manhacks came busting through. He'll die if you abandon him so give him a hand. Before heading out the door, climb the nearby ladder and take the supplies he offers you.

13. Ready your crowbar before opening this door, then bludgeon the manhacks as they fly through.

14. Flying buzz saws + flammable barrels = suicide. Back away before the explosion.

15. Grab supplies through the door at (1) before exiting through the door at (2).

16. Move quickly to avoid the CP fire. Shove the crate and barrel aside then continue through the door ahead.

17. Many Manhacks pursue. You can drop below to avoid them for the moment, but you'll want to return to the indicated ventilation grate.

18. Bust through the grate and battle a solitary Manhack to find this large cache of supplies. A CP will attack as you return from the grate.

19. Take the ladder down then defeat these CPs, taking one of their SMGs. Turn right.

20. Explode this barrel then enter the small crawlspace beyond.

21. Inside are supplies, including a grenade. Also in here is your first head crab zombie! Another CP attacks as you exit the crawlspace.

22. Farther along the cavern is this rotating...thing. Hop in and keep left.

23. Here's your way out. You'll have an opportunity to come up for air once -- take it.

24. Chances are you will emerge at the end of the tunnel facing the wrong way. If so, turn around and jump over the opening.

25. Battle these CPs, plus more in the clearing ahead, where you'll find supplies. The crate at the top of the stairs holds a medkit, but be careful of a barnacle to the immediate left of this shot. Climb onto the large pipe on the right.

26. So long as you patiently wait for the pauses between steam bursts, you can proceed along the pipe safely. When you see this lambda, ready your crowbar to defeat a Manhack and to bust through those boards on the right. Drop into another pool on the right.

27. Emerge up a ladder to find more barrel-happy CPs. Speed is your ally here. Don't engage the CPs while in the open. Instead, sprint to the right.

28. Rush into this archway, where you'll find a medkit and ammunition to the right. This is a much safer position from which to engage the CPs.

29. Defeat the CPs then open the chain link gate at the end of the archway, unleashing this large swarm of Manhacks. Back up, firing your SMG along the way.

30. Grab the supplies then smash the ventilation grate with your crowbar. Climb into the vent.

31. Drop from the ventilation shaft into this challenging room. There are countless flammable barrels and Manhacks. Shoot any of the barrels and they all go off, killing you. Instead, quickly descend the ladder on the left.

32. Drop beneath the water, ready your pistol and look up. Fire at the barrels above, exploding them all and taking out most of the Manhacks while you remain in relative safety under the water.

33. Clear out any remaining Manhacks with your weapon of choice. Note the tunnel on the left -- imagine swimming into it. If only the water level were higher...

34. Return above and find this medkit. The explosions may have knocked it to a different spot than what you see here. Return to the ladder, but this time climb up.

35. Hop onto the pipe then follow the arrows to the next room.

36. Your objective is the indicated valve. Follow the catwalk to the far corner.

37. The valve is hard to see from this angle, but I still have it marked by an arrow. Crouch, then drop to (1), then to (2).

38. Drop to the white pipe at (3), then drop to the double white pipes at (4).

39. Turn the valve and the water level rises. Smash the crate for health.

40. Backtrack to the previous room, where you'll do battle with another swarm of Manhacks.

41. Return to this now-accessible tunnel. Swim on through.

42. Come up for air, then swim to the bottom and smash these planks. The debris beneath the planks floats to the surface.

43. Hop over the debris to lift yourself to the ledge. Ascend the stairs and follow the arched corridor to load the next map segment.

44. Climb the ladder and step onto these planks. Drop to the water below and head left. Ready your SMG.

45. Battle more CPs and Manhacks.

46. Say, "Cheese" for the camera before climbing over this barrel to reach the ledge, where you'll find plenty of supplies.

47. More Manhacks attack down this archway. Your crowbar serves double-duty against the Manhacks and to clear away the crates ahead. Supplies and barnacles are at the end of the tunnel to the left.

48. To the right is this ladder. Climb to find this lambda indicating supplies to the right. Follow the planks to find more supplies before dropping below.

49. The yellow areas below are radioactive and will injure you. Ramp up the boards on the left then follow the pipe right to find supplies. Jump across the higher yellow pipe then drop to the ground.

50. Ahead is the next stop on the underground railroad. Energy and health are to the left. Ammunition is in the crate on the right. A radio transmission mentions something about an airboat...

51. Incoming shells bring head crabs. Health and ammo are to the left.

52. Peer through the fence to witness this poor chap struggling with a head crab problem of his own.

53. Meander through the debris, fighting more head crabs and then these head crab zombies. Unlike those in the original Half Life, these head crabs will continue fighting after the zombies go down, so aim for the head.

54. Time your advance to avoid these swaying wires.

55. The path ahead is through here. Grab health, energy and ammo from the supply crates -- just be careful not to touch the walls.

56. You'll soon come to this radioactive drainage ditch. Carefully walk along the wooden planks.

57. The planks lead to the woman you heard on the radio a moment ago -- and the airboat! After she opens the makeshift gate, hop on in and take it for a spin!

58. Fly the airboat over the ramp at (1). Exit and walk to the turn crank at (2) which opens the gate at (3). A supply crate and a head crab are near the crank.

59. Re-mount the airboat and sail through the now-open gate to load the next level. You can pick up supplies from an upcoming crate by maneuvering the boat directly over it.