Newmoon Island is a rocky island that has a small clearing with a pond containing darker than normal water which is shaped as a circle to represent the new moon. No wild Pokémon other than Darkrai are known to reside on the island.
To get to the island, one must receive the Member Card at a Nintendo event; this allows players to enter the locked inn at Canalave City, whereupon after going to sleep they will experience a nightmare and wake up on Newmoon Island. Due to the Surf glitch present in the Japanese version of Pokémon Diamond and Pearl, players are able to reach this area without the Member Card, a problem that Nintendo has fixed in subsequent language releases of the games. The much more complex tweaking glitch allows players to reach this area in international versions of Diamond and Pearl as well; this glitch was addressed in all releases of Pokémon Platinum.
After this area has been initially accessed, Sailor Eldritch will take the player back here as many times as liked. While nothing of interest remains on the island itself, it is one of two entrances to the northwest corner of Sinnoh's Underground, the other being Fullmoon Island.
On the Town Map, this island's shape is a mirror image of Fullmoon Island.
The Underground can be accessed at any time from Newmoon Island, despite the player falling under a nightmare and technically being asleep when activating the Member Card event and visiting the island for the first time.