Entering JONLEVEL or Temple is a Single Player level.
This is the first map ever made with THRED. This level consists of little more than a bunch of geometry. While Jim Lowell originally did not intend to release the level, he did such due to the public interest for it on Undernet.
The final section of the level, once the player drops down a hole, is a one way route and has no method of allowing the player to return to the previous areas.
Walkthrough
Head to your left to collect the Grenade Launcher, then continue forward through the tunnel ahead. Drop off to your left or right to reach the Water below, then cross the length of the room. Head downwards to reach a tunnel, then follow it and rise to the surface. You will find yourself in a relatively large room with a platform above you, head around the railing to the stairs to your left while turning left. Proceed up the staircase, then turn around and go through either doorway to reach the platform. Follow the platform to the opposite side of the room, then drop into a hole to reach the Water below. Swim to the bottom of the large pool, then move forward through a tunnel angled upwards to reach another room filled with Water. Turn right, then follow the corridor to the end to reach a tall room filled with brick.