What does the switch under the lava on "All Roads Lead To Sol..." do?

Before you ask... "what switch?"... here's a screenshot of it.

The current version of the Marathon Spoiler Guide (v3.0b4) dated Febuary 4, 1997 has this to say on the subject:

"In the same room as the laval [sic] that you have to swim across (above), there is a switch that is under the lava. NO ONE KNOWS WHAT THIS DOES! Map making programs don't reveal the purpose of this switch. In otherwords, don't waste your energy trying to hit it."

The Marathon's Story page, never one to let a mystery rest in peace, now gives you the answer.

The switch actually opens this door at the bottom of the lava pool which eventually leads to Bungie's secret credits terminal.

If you are lost... here is the room where the lava pool is located.

To get into the lava pool room however you must first smash this panel (see below) which opens the door to the lava pool room.

Smashing this panel also has the nasty effect of deactivating the switch under the lava as well as the rechargers and pattern buffers you have passed. So activate the switch first, then smash the panel and the door at the bottom of the lava pool will be open.

Ah! but surely you get the same result if you just smash the panel to right of the exit door?

Indeed you do. Smashing this panel will also open the door at the bottom of the lava pool.

So what's the point of activating the switch under the lava?

Ah! that's a mystery for another day. ;-)

Interesting to note that the most recent version of the Marathon Spoiler Guide v3.0 dated April 27, 1997 has now dropped all reference to the switch under the lava.

The Marathon Vidmasters' page offers additional insights into this level. I have reprinted parts of it here:

The level All Roads Lead To Sol... remains an enigma. Created by Jason Jones to atone for his sin of Colony Ship For Sale, Cheap it is the only solo level that cannot be completed in one try on any difficulty level other than Kindergarten.

Decide to visit the Bungie credit terminal (point A on the map below) and you cannot survive the final room (point B). Survive the final room and you cannot visit the credits terminal. In either case you must quit and restart your game.

Only on Kindergarten is it possible to survive the lava swim to the teleporter, visit the credits terminal, recharger and teleport to the final room which is now no longer flooded with lava. But why only on Kindergarten?

In Marathon if you survived the Bungie credits terminal room in Ingue Ferroque on Total Carange you could still proceed to the final terminal completing the level fully. The same is true for the final Infinity level Aye Mak Sicur. But not on All Roads Lead To Sol.... Why break with convention?

Did Jason Jones just replace one sin with another?

If I ever texture-map another axis-aligned
polygon, it'll be too soon. I'm real sorry
for being Blake, and I hope All Roads Lead
To Sol atones for the grievous sin of
Colony Ship For Sale.

Jason Jones, credit terminal on All Roads Lead To Sol...

Was it ever the intention that this level could be fully completed in one go on higher difficulty levels? Was the level finished hastily without the means to complete it? While the switch under the lava (point C) has a function it seems to have been lost in the grand scheme of things.

Never Eat Yellow Snow

In the Original Level Notes for Marathon 2 the level All Roads Lead To Sol... was to have a secret exit to a level called "Never Eat Yellow Snow". This was planned to be a throwback to Pathway Into Darkness, specifically the first level of the 1993 game. The original intent was to make the connection between the two games explicit.

In the end this plan was dropped.

Could the switch under the lava and the oddly functioning panel room be part of a more elaborate secret which involved not only the path to Bungie's credits terminal but also the exit to "Never Eat Yellow Snow"?

Whatever the answer changes were made to All Roads Lead To Sol... which made it impossible the visit the secret credits terminal and successfully finish the level under any difficulty level other than Kindergarten. It leaves an unsatisfactory taste in the mouth and it is very unlikely that Jason Jones intentionally made it that way.

What do the 4 breakable panels in the panel room do?

Well this is what we know. Going clockwise around the panel room.

Panel 1 (the first one you see as you ascend the stairs).
This opens the first of two exit doors to the start of the level AND lowers the walkway to the 3X recharger below the lava. Yup... no more 3X shields for you.

Panel 2 (to the left of the exit door).
Well this is an odd one. It opens the door to the lava pool room leading to Bungie's secret credits terminal. But it also deactivates all switches, rechargers and pattern buffers you have already passed. Ouch... this is it folks... the only way is forward.

Panel 3 (to the right of the exit door).
This opens the second of the two exit doors to the start of the level AND opens the door at the bottom of the lava pool leading to Bungie's secret credits terminal.

Panel 4.
This floods the current area with lava and reduces it in other areas AND opens two doors allowing you to access the final part of the map.

The panel room is akin to a form a Russian roulette... better known as Marathon roulette. Choose wisely.