Diving Mirror

A diving mirror (or scuba rearview mirror) is a compact,, often convex safety tool designed to increase underwater situational awareness. It allows divers to monitor buddies, check equipment, and view surroundings without turning, which is particularly useful in caves, wrecks, or tight spaces. 
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Key Features and Benefits
Safety & Visibility: Enhances safety by enabling a quick look behind or at equipment (e.g., checking for leaks).
Convenience: Compact size (approx. 5.6-5.7 cm) makes it easy to handle without adding bulk to gear.
Wide-Angle View: Convex glass lenses provide a broad view to eliminate blind spots.
Attachment Methods: Usually equipped with an elastic, adjustable wristband or a lanyard to attach to a D-ring on a BCD (Buoyancy Control Device).
Durability: Made with ABS or similar plastic housing to withstand underwater pressure. 
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Common Uses
Buddy Monitoring: Keeping an eye on a dive partner.
Cave/Wreck Diving: Checking on fellow divers in tight, overhead environments.
Equipment Check: Inspecting gear for leaks, particularly for sidemount divers.
Underwater Navigation: Observing marine life or obstacles behind you. 
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These mirrors are often considered a vital, low-cost (approx. $10) addition for responsible, especially technical or cave, diving.