Just to add info on my earlier post above (perhaps into first post or the link).
Check out the link in first post:
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Important archipelago moments:
- Lvl 10 Lt. Hammerman base for Tropical Dr. T
- Level 8 Radar to unlock Lvl 20 Lt. Hammerman base for Volcano Dr. T
- Level 15 Radar to unlock 2nd diving spot (volcano)
- Level 16 Radar to unlock the one and only ice island
- Level 17-20 Radar does not unlock any further regions
Regarding the above statement, turtles need Radar level 15 to unlock 2nd diving spot (volcano) at the far right. At the same time, this will unlock the left-most stone RB (2nd stone RB).
I forgot to screen capture though, the cost is cheap at 30k+ & 70k+ gold.
XP39, HQ16, Radar L15
Following the steps, the last map area still unlock is north-west of Dr.T volcano spot (ice base), as per the quote above.
Edit: added pictures. Hope this is informative enough.
When you get radar 15, first need to unlock 34k gold at right side. Notice the left side is not available.
Once unlock, the left side is available for 70k gold.
Once unlock, 2nd stone resource base