When you and Professor Rowan feel a strong earthquake while inside, you are forced to go and investigate what's going on, it turns out that Team Galaxy is trying to steal the Lake Spirit Pokémon, and it's a race against time to get there. as fast as possible. You can. Here's what you need to know about how to find the entrance to Lake Valor in Pokémon Brilliant Diamond and Brilliant Pearl.
The quickest way to get to this point is to use your Fly HM to get to Veilstone City and proceed along Route 214. You've probably been this way before, so you may encounter a handful of trainers you've met before. If you've fought them, you don't need to worry about a rematch, but if you've lost them, you'll need to stop and fight their Pokémon.
Your goal is to reach the area just before arriving at the Grand Lake Hotel. If you remember, there were a couple of Team Galactic recruits standing in front of an entrance to the left. You can now enter this area and head to Lake Valor. Here you can see what Team Galactic has been up to and try to stop them before it's too late.