Banda Sea earthquake

The Banda Sea is an area of the Pacific ocean that is surrounded by hundreds of small islands, forming the Maluku Islands of Indonesia. It is about 1000 km (600 mi) east to west, and about 500 km (300 mi) north to south.

In 1938, on February the 1st, the Banda Sea was the site of the 9th strongest earthquake in the 20th century, generating massive tsunamis. Thankfully no-one was killed in the earthquake, but given the region and the chains of tiny islands it seems perfectly suited for some deep, dark mystery.

An obvious starting point is to link this to the Dagon story and plot hooks as the region is about right. The Dagon story does take place a bit further out into the pacific, but having a sunken realm in the Banda Sea makes some sense as we have all of the islands that are the peaks of something much much larger.

If you want to avoid raising a major deity and driving your players bat____ insane, you could set your adventures in a sunken city; a bit like Atlantis. Maybe a local cult, or a group of Nazi scientists have worked out it's location and have started to collect artifacts from it; maybe it's a city full of Deep Ones that needed cleansing...

For non-mythos related adventures, it could have even been the site of a secret German/Japanese submarine base... blowing up the base could cause a major collapse of a cave system, causing the earthquake.

Dagon

The next installment of the Lovecraft related plot ideas is based upon his second short story, Dagon.

In the story a sailor from WW1 gets shipwrecked in the Pacific and ends up on an exposed piece of land with strange topography. He wanders the land looking for a way off, but instead encounters a series of alien carvings and statues. He escapes the island, completely insane, in an attempt to escape from a monstrous being that rises form the deep...

Shifting this from WW1 to WW2 is simple enough, but having the players actually step foot on the island and encounter Dagon may be a bit too much unless you want to either kill them or drive them completely insane; so what can we do with it?

1) The sailor may have brought back rubbings of the carvings, or a small statue. This artifact could lead to a number of different missions; maybe the original visit to the island was during WW1 and the artifacts made their way into the British Museum. Perhaps they get stolen during the evacuation of the museum treasures during the Blitz and the investigators need to track it down and preventing a cult from using it to raise Dagon.

2) Alternatively, the investigators may be sent to a mysterious new island in the pacific and end up trying to prevent the Germans/Japanese from raising Dagon.