Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; General Maintenance Action
Human Factors
In cruise at FL 360; relatively smooth with occasional light turbulence.  About 50 mi south of ZZZ.  Forward and aft lower cargo compartments indicated fire with associated EICAS; Master Warning; and aural alert.  Called for QRH procedure and executed an immediate turn and descent toward ZZZ while simultaneously [advising ATC].  PM (Pilot Monitoring) found appropriate QRH checklist and began execution.  Just prior to fire bottle arming step; all alerts and warning cease on their own.  We discussed and considered the likelihood that both compartments had independently caught fire and extinguished on their own simultaneously and agreed that it was very likely a malfunction.  We elected not to arm or discharge the fire bottle(s) as the situation no longer conformed to the QRH condition statements and our reasoning that there was a malfunction.  We did; however; elect to continue to ZZZ.  Upon landing on Runway XX; we immediately shut down the right engine while establishing communication with crash/fire rescue and asked them to conduct thermal imaging assessment of the forward and aft lower cargo compartments.  Both were reported as cold.  We taxied to the ramp.Upon arrival a ZZZ; ramp personnel opened the lower cargo compartments.  Ramp and maintenance both reported that they had been loaded in excess of the compartment's marked limits.  From the MX write-up resolution 'Upon arrival at ZZZ; it was noted that the FWD and AFT cargo compartments were loaded with bulk freight all the way to the ceilings'.  All system tests were normal and our best guess is that improper compartment loading led to the erroneous fire indications.  No obvious equipment deficiencies were found.Suggestions: Retraining and refreshing of ramp personnel regarding the importance of proper loading procedures and compliance with limitations.  It may be instructive to inform them of the consequences of failure to comply with procedures.  Also of note/concern was the fact that ZZZ ATC did not have any contact information for [company] operations.  Because of the time critical nature of the incident; we were not able to communicate directly with the company [Ramp or Operations]; and ATC did not have the information to do so.  I suspect that is an item that needs refreshing at more than just this one station.
While cruising at FL360 enroute in light turbulence we had a FWD/AFT Cargo Fire indication. As the pilot monitoring I pulled out the emergency checklist and the pilot flying recommended diverting into ZZZ. We [advised] ATC and started an emergency descent. After 1 minute or more the indication extinguished. We continued our descent into ZZZ and full stopped on RWY XX and had fire rescue run the thermal gun over our lower cargo compartment where the indication originated. The cargo compartment was cold; so we taxied to the ramp and shutdown and gave the jet to maintenance.Cause: Upon arrival mx opened the cargo door where the lower compartment was loaded to the ceiling. Our best guess is the improper loading was causal to the erroneous cargo fire indication. Suggestions: Ensure the ramp cargo loading team follows all directions when loading cargo. If there is a limit to height of stacking parcels to allow airflow; it is essential to follow the limitations. Also; ATC did not have the telephone numbers [for our company] so it is our recommendation to update contact numbers to ATC agencies.
B757 flight crew reported diverting due to a Cargo Fire EICAS Master Warning.