속에 and 안에 both mean "in, inside" But, 속에 has a nuance of "innermost, amid, invisible from outside" ex) 땅 속에(in the ground), 물 속에(in the water) 방 안에(in the room), 며칠 안에(in the next few days)

It would be very helpful to check the opposites of them, 속에 <=> 겉에(at surface) 안에 <=> 밖에(outside) \^o^/