Deep End
Deep End
[Caption above the scene] How deep ends form in pools
[On the left of the image is the shallowest water in the pool, about the height of Megan. All the water in the image is grey. She is swimming in the water, and a duck floatie and a beach ball are floating to the left of her. It is labeled] Shallow End
[Underneath, a thick layer is labeled] Pool Floor
[Going to the right from there, the pool floor begins to curve downwards. As the floor goes down, the water gets deeper. In the deepest area, it is labeled] Deep End
[At the bottom of the deep end, there is a curve and a deposit on the pool floor. Within the sediment and pool floor, there are some small pools of trapped water, labeled with three arrows] Trapped Water
[On the pool floor an arrow indicates that the oceanic plate is moving left-to-right across the image, which is labeled] Subduction
[Some of the water pools are dragged along by the pool floor, while others float up through the ground. The latter are accompanied by several arrows pointing up to indicate upwards movement. These are labeled:] Upward Migration
[At the surface there is an area, which is labeled] Splash zone
[The water erupts in two geysers, the left slightly larger than the other. Several children (small versions of Ponytail, Hairy, and Science Girl as herself) are playing there. Science Girl is sitting with her arms in the air facing the geysers, and Ponytail and Hairy are running towards the right geyser, Hairy with his arms in the air. The area is labeled] Splash Zone
[To the left of the splash zone is the edge of the pool, where a Cueball figure is in mid-air after jumping off the diving board, with his arms outstretched. This is labeled] Pool Deck

Pools, like oceans, contain water. This comic produces a schematic for the former, derived from science about the latter. On Earth, the surface consists of tectonic plates which move around. In this comic, Randall equates swimming pools with plate tectonics , to explain how deep ends form in said pools. In actuality, swimming pools aren't formed by plate tectonics (at least, not the ones made by people; we can't be sure about the others).
A swimming pool is a pool of water, typically used for swimming. Most of these have a deep end and a shallow end. This is intentional, usually to allow less confident swimmers to have somewhere to stand up when needed, while also accommodating activities (such as diving, underwater swimming, rescue practice, etc.) which would not be possible in shallower water.
Subduction , a geological process in which one plate slips beneath another and is forced down into the mantle, is shown here as the reason swimming pools have deep ends. This usually takes place between continental plates and oceanic plates, although it could happen with two oceanic plates. The comic depicts the former, an oceanic plate subducting under a continental one. With tectonic plates, this often results in a deep oceanic trench where one plate slides beneath the other, as well as a chain of volcanoes above areas farther along the subducting plate, where rock that has liquefied from the subduction comes toward the surface as magma and erupts in volcanoes. An example is the Cascadia Subduction Zone in which the Juan de Fuca Plate is subducting beneath the North American Plate , creating the volcanic Cascade Range .
A splash zone is an area of a waterpark with water being sprayed around, allowing people to get wet without the need to get into the pool. It is not a geological term, but splash zone can mean the area next to a coastline that gets splashed by waves. In this comic, the splash zone consists of natural geysers, fed by the bubbles of water that return upwards from the subducted plate.
The title text refers to back-arc basins , zones of depression that sometimes occur slightly beyond volcanic arcs due to a rift in the tectonic plate. The ban on running in this area likely has more to do with its proximity to the pool area than any danger intrinsic to back-arc basins.  A typical safety rule around swimming pools is to avoid running on the pool deck to prevent injuries due to slipping and falling on the hard deck.
Other comics that mention unusual tectonic plate motion include 1388: Subduction License and 1874: Geologic Faults .
[Caption above the scene] How deep ends form in pools
[On the left of the image is the shallowest water in the pool, about the height of Megan. All the water in the image is grey. She is swimming in the water, and a duck floatie and a beach ball are floating to the left of her. It is labeled] Shallow End
[Underneath, a thick layer is labeled] Pool Floor
[Going to the right from there, the pool floor begins to curve downwards. As the floor goes down, the water gets deeper. In the deepest area, it is labeled] Deep End
[At the bottom of the deep end, there is a curve and a deposit on the pool floor. Within the sediment and pool floor, there are some small pools of trapped water, labeled with three arrows] Trapped Water
[On the pool floor an arrow indicates that the oceanic plate is moving left-to-right across the image, which is labeled] Subduction
[Some of the water pools are dragged along by the pool floor, while others float up through the ground. The latter are accompanied by several arrows pointing up to indicate upwards movement. These are labeled:] Upward Migration
[At the surface there is an area, which is labeled] Splash zone
[The water erupts in two geysers, the left slightly larger than the other. Several children (small versions of Ponytail, Hairy, and Science Girl as herself) are playing there. Science Girl is sitting with her arms in the air facing the geysers, and Ponytail and Hairy are running towards the right geyser, Hairy with his arms in the air. The area is labeled] Splash Zone
[To the left of the splash zone is the edge of the pool, where a Cueball figure is in mid-air after jumping off the diving board, with his arms outstretched. This is labeled] Pool Deck
