Sheldon and Raj contemplate a sacrifice for science – sort of – in the preview, clips, and stills for The Big Bang Theory season 8, episode 6.

The unlikely duo of Sheldon and Raj will journey to the center of the earth (or a couple of feet down into a mine) when The Big Bang Theory season 8, episode 6 airs on CBS.

The preview for the episode, titled “The Expedition Approximation,” also hints at tension between Leonard and Penny while their pals are, uh, risking their lives down in the depths of the Earth.

The preview also suggests that The Big Bang Theory season 8, episode 6 will be the last episode to air on Monday nights. After this week, new episodes of the CBS comedy will broadcast in the show’s established Thursday night time slot.

The first sneak peek from “The Expedition Approximation” focuses on Raj discussing said expedition with Sheldon, who proves uncharacteristically unflappable.

We can’t wait to see what convinces the clean-freak-ish Dr. Cooper to pee in a bucket.

The second clip from The Big Bang Theory season 8, episode 6 offers an update on Penny and Sheldon’s new career paths.

We can always count on Sheldon to bring the modesty. But still photos from tomorrow night’s episode suggest that the physicist’s dignity may not remain fully intact when exposed to real dark matter… by which we mean, rocks.

The official synopsis for “The Expedition Approximation” fills us in on exactly why Raj and Sheldon will be going down this rabbit-hole, and offers more information on Leonard and Penny’s storyline in The Big Bang Theory season 8, episode 6.

“Sheldon and Raj test whether they could survive a dark-matter research expedition in a salt mine by simulating the conditions in a steam tunnel,” CBS says, “Meanwhile, Leonard and Penny seek advice from Wolowitz and Bernadette after they fight about money.”

The Big Bang Theory season 8, episode 6, “The Expedition Approximation,” will air on Monday, Oct. 20 at 8:00 p.m. on CBS.

How do you think Sheldon and Raj will handle the dark in “The Expedition Approximation”?