A cave chamber is an area that is large and open, especially in width and length. A person may or may not be able to stand in a chamber, but chambers are like rooms in a cave where tunnels open up to larger areas of space. Tham Luang has several chambers that are connected by tunnels.

Rescuers gave each chamber a number, and the boys were sheltering in Chamber 9. Water flooded the cave from Chamber 3 and beyond, so rescuers had to pass through several chambers and tunnels to reach the boys. The diagram on page 16 of the book provides a visual representation of the many chambers in Tham Luang, including to-scale illustrations of the boys’ size in relationship to the chambers’ sizes.

Just what it sounds like, groundwater is water contained in soil and rocks underground. Understanding the groundwater situation at Tham Luang is essential to understanding why it flooded so quickly. As rain fell in Mae Sai in the days before June 23, 2018, the ground became saturated, or full of groundwater. However, the groundwater wasn’t visible to the boys; they walked through the cave on dry ground.

When a particularly intense rainstorm hit on the afternoon of June 23, the rainfall had nowhere to go—the ground was already saturated.