Right Side of the Mine:
When you have completed all available floating island expansions, this side of the Mine will be locked and you will no longer be able to Mine. This is intended. If more floating islands are released in the future, the Mine will reopen.
The right shaft of the Mine unlocks at level 80. Here, players can play the Mining mini-game to find Island Parts, which are used like Mystic Maps to expand on the brand new island to the northwest of the main island, near the Mine. Unlike the normal islands, this new island is built as you expand.
Left side of the Mine:
When the left side of the Mine is changing content, it may be temporarily closed while the Dragon is switching to a new Dragon. This is intended and the Mine will reopen shortly.
The left shaft unlocks at level 45 and contains parts to craft Dragons. These parts are typically available for a limited time in the Mine. Be sure to craft your Dragon within the available time limit in order to receive the Dragon.