"The switch to your right activates..."
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 with eventually leads to Bungie's secret credit 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 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 pool will be open.
Ah! but surely you get the same result if you smash the panel to right of the exit doors.
Indeed you do!.
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 under lava switch.