Head gasket blew and coolant mixed with engine oil. It was expelled invisibly through exhaust system. The first sign of trouble was the oil pressure light and check engine light, but it was already too late. The warning lights were useless. The damage was already done. The engine was running without oil in it and I had changed the oil and put a fresh batch in just 6 weeks prior.

Because of the blown head gasket, the oil and coolant levels dropped dramatically in a short period of time. Topping off the coolant fixed temperature gauge issues. I even tried a new thermostat, but got no indicators about the engine oil, which was dropping rapidly. By the time the oil pressure light came on, it was already too late.

The oil pressure light in a Honda isn't an oil pressure light. It's a "Time for a new engine" light. It's completely worthless. It provided me with zero warning about the blown head gasket. The moral of the story is, because of the potential of a blown head gaskets, if you see any weird activity on the temperature gauge, the first thing you should do is start aggressively monitoring oil consumption, lest you fall into the same trap. I'll never buy another Honda. Done.

Update from Mar 8, 2018: It wasn't a blown head gasket. It was a cracked engine block. Honda covered the cost of the repair under a technical service bulletin and an extended warranty out to 10 years for this particular problem. Offered for 8th gen civics from 2006 to 2009.

Car began to knock loudly, pulled off to roadside. Motor not restart. Towed to nearest service station, mechanic said no oil drain plug, all oil had drained. Honda Customer relations by phone from Torrance, CA said warranty does not cover, as aftermarket parts found. I pointed out that the Federal Trade Commission had issued a consumer alert that it is illegal for Honda to deny warranty on grounds that aftermarket products were used, unless Honda could prove those parts caused the malfunction. Honda then changed its finding, thusly: "No oil drain plug was found in the vehicle. Lack of oil was caused by the drain plug not being in the vehicle, and because of that the damage was not caused by a defect on the vehicle." This is a direct quote from Honda's Customer Relations rep via phone. I was told that a Honda investigator on the scene was from the Southern Florida region, and that he personally examined the car. I am presently discussing this charade with federal regulators in Washington, DC, and relevant oversight committees of the US Congress.