The Anthell is a series of twisting and turning tunnels big enough for several men to walk through shoulder-to-shoulder deep below the Route 10 forest floor.
Carved out by the mandibles of thousands of bugs, the Anthell is the largest bug-nest in West Tandor and old legends suggest that it may be the birthplace of all Bug-type Pokémon in West Tandor. The upper levels of the nest are part of the major land route along Route 10 to Amatree Town; even the uppermost level goes far enough below ground to allow humans and Pokémon to travel beneath the river that cuts through the middle of the Baykal Rainforest. Foraging Smore, Sponee, and Tricwe are frequent sights on the upper levels, but once you begin wandering deep underground, their dangerous evolutions prowl the tunnels, ready to repel any intruders.
Rumor has it the in the very deepest depths of the cave lives the mother of all bugs, a giant, vicious queen that only awakens once every few years to feed — the Seikamater. Of course, those are just legends...
To get into the deeper Anthell, you must first talk to the man in front of the Amatree Town Gym. He gives you the Bug Spray, which can be used to clear out the Firoke, Sponaree, and Harylect that guard the entrance. Once you descend into Deeper Anthell, you cannot return to Route 10 until you find Tiko (except by using an Escape Rope or Dig), and you cannot return to Deeper Anthell after defeating the Seikamater. Make sure you bring enough items to make it through the end of the tunnel.
After speaking to the Amatree Town elder blocking the gym and receiving the Bug Spray, go back into the upper tunnels on Route 10. When you reach a branch of the tunnels blocked off by a mass of bugs, use the bug spray to send them scrambling into the darkness.
A short way down the newly cleared path, you'll find a dark open hole in the ground to Deeper Anthell. The lower levels of the Anthell are swarming with Sponaree, Firoke, Harylect, and missing trainers whose minds have been absorbed by the hive.
After travelling through the paths, you'll eventually find a second open hole. Dropping through this hole will land you right at the entrance to the Seikamater's nest. You'll find Tiko wrapped up in a web with bugs scuttling on either side of the path. Talk to Tiko to free him and he'll ask you for help in defeating the queen.
The Seikamater is level 40 and cannot be captured, attempting to do so will give the message "For some reason this Pokémon can't be caught!" Once you defeat her, you will obtain the Royal Jelly and you will be teleported back out of the the Anthell. The Royal Jelly, when held by a female Smore, Tricwe, or Sponee, will cause any one of them to evolve into a Seikamater when they grow to level 45 or higher.