From Cyan Today:

Welcome back, again,

Everything takes longer than expected these days. Cyan Worlds' plans are to
move MO:UL to open source, and we finally have some good news. We've taken a
first step and started a MO:UL server, so the Ages of Uru are available
again. We've opened all the Ages, and added most of the goodies in MO:UL. We're
referring to it as MO:ULagain-feel free to explore and enjoy.

So the Cavern Returns: it is available for download at the main MOUL site, just be patient many ppl waiting to get in.

the servers are at 100 percent as people line up to get back in, the forums
are buzzing and many happy ppl have waited a long time for this. Very strong
faith in Cyan and their wonderful work.

This will be the MOUL that was online with GameTap in 2007-2008 with only a
few things missing. All the core Ages plus new ages added with MOUL. There
were in game events with historic characters from the URU storyline the DRC
council to start things off.