Danger: Diabolik

After an armored car leaves the bank with ten-million dollars, it is attacked on route by Diabolik (John Phillip Law) who manages to steal the money and escape with his partner Eva Kant (Marisa Mell). Leaving the money in their underground hideout, Diabolik and Eva attend a press conference held by the Minister of the Interior which they disrupt by releasing laughing gas. Due to the high level of crime, the death penalty is brought back. The police cannot find Diabolik but gang leader Valmont (Adolfo Celi) suffers at their hands because of a clampdown due to Diabolik's actions. Realising things can get only worse because of Diabolik's crime spree, Valmont contacts Inspector Ginco (Michel Piccoli) and makes a bargain to catch Diabolik alive for the police. In his hideout, Diabolik decides to steal the famous Aksand emerald necklace for Eva's birthday from the Saint Just Castle. Valmont builds up an identikit picture of Eva and circulates it as a means of capturing Diabolik. Diabolik scales the walls of Saint Just Castle where he finds the police are waiting, Diabolik manages to steal the necklace by fooling the police officers with mirrors on the road and dummy decoys of himself.
Valmont has made further plans to lure Diabolik out of hiding by having one of his henchmen kidnap Eva. To rescue her, Diabolik boards Valmont's airplane, along with the ten million dollars and emerald necklace as a trade for Eva. Diabolik is ejected from the plane but manages to grab Valmont just before his plane explodes from a bomb that Diabolik had placed earlier. Diabolik manages to rescue Eva as the police close in on them. Eva makes her escape as Diabolik kills Valmont, is trapped and takes a golden capsule. The police find Diabolik and proclaim him dead. Later while the police are holding a conference on the death of Diabolik, he is about to be autopsied when he returns to life. Diabolik reveals that he has faked his death through a technique taught to him by Tibetan lamas. However, If he does not get the antidote within 12 hours, he will die.
Posing as a nurse, Eva wheels Diabolik past groups of people where Diabolik sneaks into a morgue where Valmont's body has been cremated. Diabolik collects the 11 emeralds from Valmont's ashes and escapes. The police find out that Diabolik is not dead and offer a million dollar reward for Diabolik. Diabolik reacts by blowing up tax offices. Without tax revenue, the country is in debt and sells off some gold to buy currency. The 20 tons of gold is melted into one solid block to make it difficult to steal. Diabolik and Eva divert the train by leaving a burning truck on the tracks having it re-route to a bridge track where a bomb is placed. After being triggered when the train arrives, the gold falls into the water below where Diabolik and Eva collect it to return it to their hideout.
The steel casket containing the gold had been irradiated, which allows the police to trace it. They close in on Diabolik who is melting the gold, to put it into ingots. Being shot at by the police, Diabolik is unable to control the heating of the gold which leads to Diabolik and the cavern being covered in molten gold. Again, Diabolik is believed to be dead, with his heat-proof suit now covered in hardened gold. The police seal off the place to later recover it. Eva is allowed to pay her respects to the dead Diabolik, who winks at her.

Question: What is the cavern covered in?
Answer: molten gold