Gather a letter and a number from this cache to find the Bonus cache.
To find this cache: You'll need to think in three dimensions. And be grateful the cache isn't hidden in that pesky ivy! Good luck.
Caches along the Phoenix Trail:
Thame Moreton Circle - 01*, GC01-West Ender, INDUSTRIAL REVOLUTION 1 (short detour)*, GC02-Not the Ivy!, GC03-Rocky Moment?, GC04-Sleeper Cell, twixt Thame & Towersey*, GC5-Beware the Rabbit, GC06-Classic Hide, Old SideTracked - Towersley Halt*, GC7-Rest Awhile, GC08-Off Track, GC09-Yumcrete , GC10-QEF?, GC11-Tree Cover, GC12-Stand Sit Jump!, GC13-Reflective Space, GC14-Lost Signal, GC15-Blasted Thing!, GC16-Eclipse, GC17-First Class, GC18-Rooky Error, GC19-Isolated Location, GC20-Bridge Hand, Virtual Insanity*, GC21-Gated Community, GC22-Antlers, GC23-Front Door, GC24-Around the Bend, GC25-East Ender, GCB-Bonus!
*Not part of the Geolympix Cyclerama series, but you may pass close-by along the way.