Mona the Vampire - Series 4 - Atlantis at Last/Invasion of the Shadflies

The cartoon adventures of a 10-year-old girl who solves ghoulish mysteries. Neptune, god of the seas, discovers the long-lost continent of Atlantis right under Mona's town, and wants to restore it to its former glory. Then, Mona is suspicious after there's an unseasonable early appearance of shadflies.