That cave is for the quest where you must find the buried relics to unlock a chest of jewels and such.
Find the relics and place them on the pedestals.
Good luck.

The cave is for Relics. You can find Relics in Claim Sites around the Island. They cannot be destroyed by Dynamite, but I'm not certain on Mini-Nukes. Bring the Relic's to the Cave and place them on a Pedestal. I'm not positive, but I believe if you bring them all (and don't use Creative Mode or any MOD's), you get a Steam Achievement and a reward...

Chinese translation, care of Google Translate:
洞穴是用来存放遗物的。你可以在岛屿周围的领地中找到遗物。它们不会被炸药摧毁，但我不确定迷你核弹是否能摧毁它们。把遗物带到洞穴，放在基座上。我不确定，但我相信如果你把它们全部带走（并且不使用创造模式或任何MOD），就能获得Steam成就和奖励……

Nukes can't even scratch the relics.

There are 6 Relics to collect, these are buried in the dig sites. You only require 4 to complete the task. Put each Relic on a pedestal in the cave and you will receive a treasure chest. There is a guide if you search on You-tube that may be helpful. (Hydroneer 2.2.5 Relic Location Guide (Spoilers)).