What is ABT referring to in the context of diving?

The term ABT refers to Assumed Bottom Time in the context of diving. This concept is critical in dive planning and safety because it represents the estimated time a diver intends to spend at a designated depth before beginning the ascent back to the surface. By calculating this time, divers can manage their gas consumption and decompression status effectively.

Assumed Bottom Time helps in planning the dive profile, allowing divers to set their turnaround points and decompression stops based on their planned activities underwater. Understanding ABT is essential for ensuring that divers do not exceed their no-decompression limits and thus avoid the risks associated with decompression sickness.

The other options do not accurately define ABT. Amount of breathing time and time underwater until ascent do not specifically capture the concept of estimated duration spent at the bottom of a dive, while air bubble trajectory is unrelated to dive time management.