Invaders
tie tucked in episode

Typical Control Room stuff. Chip is asked to channel all data to the computers for chart revision. Nice posterior shot as they're hit by yet another seaquake. Asked for printout of some sort by Nelson. Yet another indication of his ability with the computers.

Maneuvers Seaview above divers.

Via intercom, verifies for Lee that all systems are secure and ready for use of liquid nitrogen.

Having cup of coffee with Lee, sitting on corner of Lee's desk, they're discussing the alien's reading for 32 hours. As Chip relates the variety of things the alien is reading, Nelson walks in, smiling at the two.  The three head off to Control Room. 

 
Lee and Chip are discussing their probable location. Auto-navigation says one thing, their common sense says otherwise. Lee asks Chip if he's rigged emergency power for the electron microscope and Chip tells him it should be ready in a couple hours. To find their true position, Lee uses sextant to 'shoot the sun' while Ski focuses the periscope on the sun and Chip calculates position from the readings. They discover that they are back at the underwater ruins where they found the capsules.

When contaminated air is found in air revitalizer, Chip calmly and with determination volunteers to go in, Ski looks at him a moment and says he volunteers too. Nelson tells them they have to be sealed inside until the contamination is found. Chip replies with a quiet, "Yes, sir."
Think that Ski is impressed with the Exec's willingness to take on the dangerous task.
Neither Nelson nor Crane is very happy.

Chip goes down first, carrying a small tank of air, followed by Ski with a tool box. The two enter the chamber and see gas coming out of system. Chip calls Control Room and Lee has area sealed off. Chip hands Ski the oxygen mask. Ski asks where Exec's air is and is told there is only one. Ski says they'll share the air. Chip tells Lee that there is an explosive gas inside (not sure how the heck he knows this unless it has a distinctive smell) and that the hatch to the air revitalizer has been welded open. They only have about 4 minutes of air. Nelson thinks they can get the two out by nosing down Seaview, forcing the gas away from the hatch. While Lee returns to Control Room to carry out plan (he is very *very* ticked at the alien), Nelson remains by the sealed area giving Chip and Ski verbal support over the intercom. Ski goes out first, followed quickly by Chip. 

Chip and Lee set up oxygen tank in Control Room to feed into Nelson's cabin via conduit set by crew as crew seal Nelson's cabin.