Molnár János “MJ” Cave (Budapest)

Intro

As I mentioned in my overall Trip Report, I don’t consider myself a cave diver but I do enjoy diving them occasionally and find the geological history and the underwater formations of different caves in different regions very interesting.

We spent four days diving the ” MJ” Cave and did a total of 5 dives. This report will provide some historical information about the cave itself, a review of the different dives we conducted, how the dive operation works, photos & videos from the dives, and some cool maps of the cave.

MJ Cave | Location

I find it fascinating that there is an entire world under the city of Budapest. I found one research paper from the International Journal of Speleology [1] which had a map of the major cave systems in the region. The MJ cave is shown in the lower right part of the map, very close to the Danube River.

According to the research paper,

The Molnár János Cave (MJC) is one of the largest known active phreatic hydrothermal caves of the world. The underwater cave, which is connected with the thermal spring of a nearby spa, was explored in 1972. The first map was drawn in 1984 by Kalinovits. In 2002, new submerged passages were discovered, and now the total length is more than 6 km.

International Journal of Speleology, May 2015

The paper also has a diagram that shows how the MJ Cave is largely under the water table compared to the other cave systems.

In addition to the above maps, the dive center has a couple maps that show how the cave snakes underneath the Buda side of the city. I also tried to create a map from Google Maps showing the approximate boundaries of the cave system.

The really interesting thing to think about when diving in the cave is that you are literally swimming under houses, pilates studios, hospitals, etc.

MJ Cave | Dive History

Most of this information is based upon descriptions from the dive center and an “interview” I conducted while on-site and some information from the MJ Cave website [2].

The cave was first discovered in 1854 by János Molnár who was a pharmacist. He found a dry cave above the current site and took a water sample and compared it to the small lake near the dive center and realized that the two were the same composition so he concluded that the lake must be fed by an underground source (note the pipe in the picture on the left that presumably feeds the bath house across the street).

In the 1950s, the first divers started to dive the cave. They would climb over the pipe that fed the baths across the street and go into the cave system. It seems as if the maximum penetration distance was about 150m (500 feet) and the total distance of all passages was about 400m (1300 feet / quarter mile) and the maximum depth was 37m (122 feet) deep. The diving continued throughout the decades following but was largely unregulated.

The first entrance was man-made and is shown in the photo below and is still functional today (although it isn’t the primary entrance).

Above the dive center and up on the hill is a building that used to house workers and there is the Lukacs Bath across the street.

In 2003, they noticed that the thermal baths across the street were getting a lot of silt in the water. The divers went into the baths and noticed the silt coming through a wall and figured there must be something behind that wall. They hammered away at the wall and ended up breaking through and finding more cave (which is now where parts of the “B” Main Line are). They continued to explore the caves and found an air pocket where the current dive platform exists. However, there was no connection between that “room” and the current tunnel where the dive operation exists today.

The tunnel that houses the dive operation today was built in the 1970s and 1980s to connect the building on the hill that housed the miners and workers to the Lukacs Bath across the street. The idea was that they would build an elevator from the building down into the tunnel and then they could take the tunnel to the baths. The elevator actually never got built (and the tunnel was actually built in the wrong direction which you can see when you stand outside and compare the direction of the tunnel to where the building on the hill stands). The project was abandoned at some point.

Divers found the room with the air pocket mentioned above in about 2010 and they realized that it was pretty close to the tunnel that already existed but there was about 10m / 33 feet of rock in between the two. They were lugging all their gear through narrow passages to get to that spot and realized it would be much easier if they could enter through the large tunnel that already existed. One day, a connection magically appeared between the tunnel and the large air pocket. Nobody quite knows how that happened. They call this the “magic staircase” that happened overnight. Of course, it is all here-say.

Diving the MJ Cave

At some point, somebody died while diving the cave and then the government stepped in and regulated access. All access today is through MJ Cave and must be guided. They have spent significant time & resources putting in a very organized line system with clear markers, main lines, exit arrows, etc. It is somewhat different than a “normal” cave in that you don’t really need to put in any “jumps” since they have laid permanent lines between the different passages between the main lines.

Below is a picture of the cave map:

The day usually starts with getting your gear ready to dive and then you meet with the guide and do a detailed dive briefing and set a time for getting into the water. I recorded the dive briefing for our last dive (which was pretty interesting as I didn’t make it through the last small passage). 🙂

Note that this video is six minutes long but gives a really good example of a typical dive briefing. Most dives in the cave are not this complex. 🙂

After the briefing, you take your bailout / deco tanks to the platform and lower them into the water. You can jump into the water or go down the steps and then put on your other tanks. Below is a 20 second time-lapse video of the process.

They have a nice deco station setup at 6m / 20 feet for the last deco stop. Ben took a couple pictures which provide a good sense of infrastructure they have set up:

One thing to note is the water temperature. There is a pretty big thermocline at about 10m / 30 feet. The graph below shows the temperature on the blue line. At depth it is about 68F but above the thermocline it is a balmy 80F. This can present challenges when diving so pick your undergarments carefully depending upon your thermal tolerance and realize that deco will be at 80F. Also keep in mind that you will likely be swimming and active during the bottom portion when it is cooler.

Dive Summary

Below is a brief description of the different dives we did in the cave along with some pictures (from Ben Lair) and some photos & videos.

Day 1 / Dive 1 & 2

Our first day, we stayed relatively close to the exit but explored quite a bit of the cave. There was a LOT of up & down in the cave. Our first dive was about an hour and the second dive was about 90 minutes. The dive profile below looks like an EKG chart!

Day 2 / Dive 3

On the second day, we did one long dive that was about two hours long with about 20 minutes of deco. We went down almost to the end of the “A Line” and got to about 550m / 1815 feet into the cave. It still had a few steep up & down segments but better than the previous day. For all those people watching on a mobile device, here is a vertical video of swimming through the cave:

I had also taken my Insta 360 into the cave on this day and shot some 360 video. Below are a couple two minute 360 degree view videos. You can move your mouse (or your mobile device if you are watching on the YouTube player) and rotate the video around to look forwards, backwards, up, down, etc.

And another 360 degree video:

The photos below are all taken by Ben Lair:

Day 3 / Dive 4

This was probably the best dive I had in the cave. We went down the A Line and then cut over to the B Line and explored a lot of areas. Nothing too tight but had some really nice passages and areas. We ended up with about 30 minutes of deco.

The photos below are from Ben Lair:

Below is a one minute video I shot from a GoPro

Day 4 / Dive 5

This was a really interesting dive. 🙂

The dive brief for this dive is shown in the video referenced above. If you forward the briefing video to about 5:20, our guide starts talking about going down vertically into a small hole. I had forgot about about the “down” part and was the fourth person to enter the space at which point it was completely silted out.

I followed the line to a break in the rock and couldn’t see a thing. I kept feeling for where the line went but couldn’t figure out which way to go. I tried to wedge myself up a few times but couldn’t make any progress. I backed out and figured that our guide would come back to find me which he did, and then went back and got Ben & Dave and we all met up and went to exit given that they were deeper during this fiasco and had racked up some extra deco. All ended well.

Below is a short video of us making our way through the cave:

This video shows me waiting to go through the area where I forgot that it went straight down and not making it through. You can get an idea of the slit storm that happened. 🙂

Below are some photos that Ben Lair took. A few of them give you some idea of the tight spaces we went through. 🙂

References

[1] Karst porosity estimations from archive cave surveys – Studies in the Buda Thermal Karst System (Hungary) on ResearchGate

[2] The story of the Molnar Janos cave

AI Transparency Statement

There was no AI used in the post other than maybe some photo clean-up work done by Ben! All words were written by me and any mistakes are mine and mine alone.