Dreamcass Island, according to Chapter 10 is an island on the edge of Planeptune. It is home to the Dream clan and rare exotic animals. Underneath the island is the ancient Kingdom of Seturn. The goddesses use this island as their vacation retreat. In return for being allowed to use the island, the goddesses help the Dream Clan.
The island's most notable feature is the mountain with the Dreamcast Logo in the center. Going down the mountain is a forest. That leads to the coast line with sandy beaches and a beach house.
Anime & Manga
Hyperdimension Neptunia: Megami Tsuushin
This island first appears in Chapter 10 with all the goddesses vacationing there. Noire spikes a beach ball away and Neptune runs off to get it. A Dream Clan guard tells her not to as there are tons of holes dug up by the ancient giant earthworm.
Blanc is curious about this and Neptune has already run off. Eventually as the day winds down, Nepgear notices her sister missing so goes out to look for her. Nepgear falls down a hole and meets Vee Seturn who looks like her sister. The two navigate the underground cavern and are able to reunite with respective sister. They find the end of the cavern and stand on a switch that should return them to the start.
The switch is a trap door to a river that does lead them back to the surface.
The goddesses vacation later in Chapter 33. Nepgear and Uni help out at the beach house, while the CPUs catch seafood for the house. When they return to the beach house, ASIC has taken control of it and taken the Dream Clan and the candidates hostage.
With creative use of their seafood, ASIC was defeated and the goddesses have both a good catch and a happy time for the reminder of their vacation.
According the Dream Clan, long ago on Dreamcass Island was the Seturn Kingdom. It was a prosperous kingdom until some unknown diaster caused the kingdom to sink into the ground. The kingdom's princesses, Vee Seturn and Hai Seturn managed to protect everyone as they fled to the surface.
In addition to that, there are holes on the beach dug up by the Ancient Giant Earthworm from the eve of distruction.