WHAT comes to mind when you hear the phrase ''a lake within a lake?'' Taal Volcano, of course, which has been referred to as that small but terrible volcano in Batangas.

From Tagaytay, you can get a good view of both the lake and the volcano.

Taal is different from other volcanoes like Mt. Mayon or Mt. Pinatubo, not only because of its size but also because it is an underwater volcano. Only its top portion can be seen.

My first visit to Taal was in August last year, with the freshman geology block of the University of the Philippines. We went to several eruption sites including the Main Crater.

At the foot of the volcano, visitors may ride a horse to the top. But since my friends and I were reluctant to shell out P300 for a bit of comfort, we decided to just walk all the way up. Besides, being geology majors, we're used to very, very long treks--no sweat! Also, traveling on foot is so much more fun.

It took us about 45 minutes to reach the rim of the crater, and along the way we saw steam coming out from little cracks in the ground. But it was nothing to be alarmed about, it being ordinary volcanic activity.

When we reached the top we were greeted by enterprising residents selling ice-cold water, soft drinks, even beer!

We stayed only at the rim. We did not go down to the Main Crater Lake. For me, it was enough that I saw it.

Last Sept. 5 and 6, I went to Taal again, with my volcanology class. That time, I went all the way down to the Main Crater.

We logged in at the Phivolcs station in Barangay Buco, which has a scale model of the area and a photo display of various types of volcanoes and volcanic eruptions. There we got a banca that took us across Taal Lake to the different parts of the island.

The first day wasn't so exciting for me as I had been there before. We looked at lava flows from the 1969 eruption and made our way up sandy slopes to see other eruption centers.

At the top, we could see Mt. Maculod. Directly behind us was Tagaytay. Spread out below was the lava field--a black mass more than seven feet high with a jagged surface and surrounded by vegetation.

We spent the night at another Phivolcs station in Pira-piraso, located at the foot of the Main Crater. The next day we set out for the Main Crater.

Hiking can be hell when there are no any trees along the trail. The scorching sun was almost unbearable. I was afraid my water supply would run out before I even reached the peak. But I forgot my exhaustion as I looked down at the Main Crater Lake. Just like the first time, the view from the rim was amazing.

And it got better as we started our descent. The water was so blue, and everything around was so still. It was like a haven.

Occasionally, we'd rest under a tree along the trail or stop to look at interesting rock exposures. And as we were going down, we could see fumaroles (hot springs) in the distance. Halfway to the lake, we came face to face with a herd of cows resting under a cluster of trees.

After what seemed like eternity (actually almost two hours), we were at the edge of the lake. The water was lukewarm and the smell of sulfur was stronger than ever. The odor reminded us of the same thing--UP's Chemistry Pavilion!

I thought: Water this acidic could never sustain any kind of life. Guess what? We found algae growing in the water. You might think: Okay, but surely you can't swim in this kind of water, right? Wrong! In fact, our teacher, Dr. Eddie Listanco, took a dip in the lake. Sulfur has some properties that are good for the skin, and if Sir had come out a skin tone lighter, we would all have jumped in.

We had our lunch by the lake. Those who still had the energy to survey the area took a closer look at the fumaroles. I was among those who didn't want to move. Instead of going around, I decided to call my Mom on a cellphone and tell her where I was (yes, there's a signal even inside the crater).

So what's it like being inside the crater of a volcano?

Well, it's pretty amazing. It was very quiet and peaceful. Hard to imagine that such a serene environment could be the site of very violent and destructive eruptions.

The hike back up and out of the crater was difficult, specially at high noon. As always, most of the boys in the class took the lead. I was doing my own pace, and the other boys and girls lagged behind.

We were nearing the peak when I caught up with my teacher and three classmates who were looking intently at the fumaroles again. It seemed Mother Nature wanted to bid us farewell with a little ''show.'' One of the vents was emitting more steam than the rest, and, to our amazement, started spewing mud.

By this time the rest of the group had arrived, and we all paused awhile to view the spectacle. It would have been great if we were still down there to see the ''eruption'' up close.

In the banca, I was so tired I fell asleep. And as I dozed off, I wondered when and where my next adventure will be.