I didn't know about this until I saw Mo's Server Thing was running it; luckily Spike was able to point me to www.whitestonetech.net/tmon/ where Tmon had it available for download, along with just about all the other new maps.
For an arena called "Underground", it is all very light and spacious, with even blue skies through some windows, and some really fine stone textures and paints, the credit for which Tmon gives to his dad, Tux_the_Penguin, who was also in charge of most of the technical stuff. There is a bounce-pad, which Tmon and his father had not yet been able to give its own colours to (it just appears to be a circular depression in the stone floor); there are all the weapons, I think, and the doubler and megapower... two of the latter, one hidden away in a secret compartment it would probably not be sporting to say much about (Sean showed me). With generous health, and plenty of rooms, all beautifully lit; this looks like a fine place to visit. There is an obvious trap, baited by one of the arena's two whompers; jump down to get it and you couldn't jump out again! Actually, it is worse than that, the bottom is false and you fall straight to your "death"! But otherwise, a nicely judged place to play. We really are being spoilt for choice, these days!