Abbreviated quotes from Wikipedia:

Mount Pinatubo is an active volcano located on the island of Luzon, at the intersection of the borders of the Philippine provinces of Zambales, Tarlac, and Pampanga.

The volcano's eruption in June 1991 produced the second largest terrestrial eruption of the 20th century (after the 1912 eruption of Novarupta) and the largest eruption in living memory. The colossal 1991 eruption had a Volcanic Explosivity Index of 6, and came some 450–500 years after the volcano's last known eruptive activity.

The effects of the eruption were felt worldwide. It ejected roughly 10 billion metric tonnes (10 cubic kilometres) of magma, and 20 million tons of sulfur, bringing vast quantities of minerals and metals to the surface environment. It injected large amounts of aerosols into the stratosphere—more than any eruption since that of Krakatoa in 1883. Over the following months, the aerosols formed a global layer of sulfuric acid haze. Global temperatures dropped by about 0.5 °C, and ozone depletion temporarily increased substantially.

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After 18 years, thousands of Filipinos and foreign tourists trek to the crater of Pinatubo each week. You start your journey from Manila to Barangay Lubigan, San Jose, Tarlac. After completing your registration at the Eco-Tourism Park, you will ride a 4x4 transport for about an hour of dusty and rough roads. Then you start walking for about an hour and half through large boulders and small creeks before you reach the crater of Mt. Pinatubo.

Fees paid at the Eco-Tourism Park is around Pesos 1,100 per person which includes the 4x4 transport, guide and porter for your bags.

Check with your heart doctor first before making the adventure.