Okay, I've got that marked in and I've seen the light, but now that I've seen and heard it after several sweeps, what do I do with it, considering where following the actual path of the shell takes you?

When the symbol shows up walk into it.

Oh, goody. Another 'specifically narrow time frame' type puzzle. At least this one isn't that bad. And you have to walk into it, and not just be there when it forms, right?

Edit: And now I've completed Into D'NI. If only you could explore more in URU. (I've played Myst V, and it felt...lacking.)

Congrats ;)

Yeah, smaller Ages (and a bit contrived story wise i thought). In comparison URU is massive in scope and it is a shame that things didn't worked out as Cyan intended, i too would have loved to see more content, more locations open in the city and so on.

It's a great ride though, one worthwhile revisiting once in a while

Wow. This thread completely spoils the ENTIRETY of To D'ni ... which is unfortunate and should be tagged.

For any further inquiries, I would suggest Prima's strategy guide for Myst URU: Complete Chronicles. It is wonderfully written in a "journal" format from the perspective of an explorer working through the adventure. Not only does it cover both To D'ni and Path of the Shell, but it also contains a wonderful full color retrospective section on Cyan Worlds, the Myst series and development of Uru and Myst Online: Uru Live.

Mind that the strategy guide covers the SINGLE PLAYER expansions to Uru: Ages Beyond Myst, as opposed to sister content that exists in the online game that differs greatly from the single player expansions.

Prima's guide for Myst URU: Complete Chronicles is available for around $30 new and less than a dollar used from online vendors through Amazon.com at the time of this post, and appears to be in plentiful supply.

Of course, you can always search online for guides, but be sure you're looking at guides for Complete Chronicles rather than guides for Myst Online.

The only issue with Prima's Complete Chronicles guide is that it only features a guide for the base game, Ages Beyond Myst, in a brief list format step-by-step walkthrough of each level. If you want a more detailed walkthrough of ABM, Prima's separate guide for the base game also comes highly recommended.

Of course you can always work out the challenges yourselves, but everybody gets stuck every once and a while.