Buried Relic is probably what you are asking. If so, then after you cleared Stormy Sea, check your mailbox (clean it up or you won't receive any mail) and there will be a mail about the discovery of Buried Relic. Go to Pokemon Square, talk to Lombre then Shiftry. You are then granted access.
Regirock, Regice, Registeel will be on 15F, 25F, 35F respectively. Their parts which is dropped after they are defeated will automatically combine into Music Box, which will make Mew appears throughout 36-98F. Music Box is necessary to recruit the 3 Regis.
There are HMs on 45F (Rock Smash) [Key], 60F (Strength) [Wall], 70F (Flash) [Key], 80F (Cut) [Key, Water, Wall].