Solen Lair Maps & new info posted on Stratics:

[Dor of Sonoma]

It seems that the green thorns are proving to be more versatile by the minute, as planting one in the desert now enables you to reach a formerly inaccesible underground area:

http://uo.stratics.com/content/atlas/solenhive.shtml

I sure wish that I could explore it this weekend, darn it...as it is, I won't even be able to start cross-breeding till tomorrow. If anyone comes up with a spare new resource seed, I will happily trade for it!

*trudges away to another 12-hour Faire shift*

[Hwarang]

Okay, just edited cause I found out my answer. Still have another question though.. does each person that goes in need to use a green thorn, or can a group of people use one, and all go in?

[Thalia Soulrift]

One person uses the thorn and a whole group can enter. The hole doesn't stay open long.