Planes were used as sub spotters and the Tin Cans (Destroyers) dropped the depth charges. Most of us has seen Das Boot.
Could a BAB (big ass bomb) like a bunker buster take out a sub?
Probably not the most environmentally friendly approach. But setting off a dozen depth charges isn’t either.
You mean currently, or in World War II?
Because currently, submarines tend to operate hundreds of feet below the surface, and depth charges are simply no longer used. Instead, most navies have heavyweight and lightweight torpedoes to target submarines. Heavyweights would tend to be launched from other submarines or surface ships. Lightweights would be dropped from planes or attached to rockets. Either torpedo will have sophisticated guidance systems to find and home in on a submarine, but the days of just dropping explosives in the water and hoping you get close are all history.
Now, if a submarine were to be found in shallow water for some reason, sure, one could drop a bomb. But bombs are not part of the loadout for anti-submarine warfare aircraft, so there would probably be a complex system of an ASW aircraft directing an attack aircraft in… but I can’t see why that would really be done, since the ASW aircraft would almost certainly be carrying torpedoes if this occurred during some sort of conflict.
They still are, actually. Sort of. I don’t think anyone still uses the primitive depth charges used in WW2, where you had to manually set a depth of explosion and saturate an area, but AFAIK the Brits and the Scandiwegians still use modernized versions of the vintage Hedgehog, which is the same concept albeit fitted with a proximity detonator (presumably featuring magnetic thingamabobs to not blow up in proximity of the ground or, say, a whale). They’re also somewhat smaller, and are fired from mortars so it’s easier to cover an area with them than having to do donuts at full speed, dropping charges along the way and ruining one’s passive sonar capabilities in the process. The advantage over a torpedo is that the travel time is much shorter, which leaves less time for the sub to counter-attack or evade.
Oh, and as of the Cold War the US, USSR and the UK also had nuclear depth charges. Presumably in case they ever needed to teach a really large bit of the ocean a lesson. I don’t know if those still exist or are still deployed in any capacity however.
As for WW2, since submarines would typically cruise on the surface back then, then yes, bombs were commonly used as ASW weapons. Not even big ones, since the plan was to catch the steel coffing with its squarepants down and make a dent in it before it had a chance to dive very deep. Of course, considering the precision of dropped weapons back then it didn’t succeed very often, but it gave RAF pilots something to do and besides, just forcing a U-boot to dive and harmlessly crawl underwater for a couple hours was a worthwhile result.
I imagine deep water would cushion the effects of surface bombs. But those bunker busters we used in Iraq would probably still give a bang pretty deep in the water.
I see the point that guided torpedoes would still be the best method.
For sure. The US Navy deployed the RUR-5 ASROC system, capable of launching a torpedo or a nuclear depth charge (which could absolutely ruin a sub’s whole day).
But you first have to find the sub. And submarines work extremely hard to be very, very difficult to find - it’s pretty much their major reason for existing.
Good point – I was really thinking of the US Navy and I spoke too broadly.
It seems like modern subs couldn’t take a pounding like the old WWII era? Modern subs are packed full of electronics and other delicate equipment.
Maybe it doesn’t matter since guided torpedoes score more direct hits? Depth charges aren’t pounding subs until they burst like in WWII.
In WWII, how close would a depth charge have to be to cause serious damage?
WW2 subs were also packed full of “electronics” and delicate equipment ;). It was just bulkier and their electronics were made of cogs. But there’s really few parts of a WW2 sub that could take a hit and it not be a major issue. For one thing, the space between the pressure hull and the hull hull was also where they stored the fuel, so most every hit meant leaking the boat’s lifeblood.
That being said, modern subs are also much bigger than WW2 ones. For example the type VII, made famous by Das Boot, was ~60 meters long, ~6 meters wide. A Los Angeles-class submarine is 110m long, 10m wide. Strategic missile subs are even bigger. Larger size means more individual pressure compartments to combat flooding.
I also expect they’re simply made to be more sturdy than German U-boote built in a rush with substandard steel. But then again I wouldn’t really want to be aboard a modern sub running into a WW2 depth charge or forgotten mine either 
I happen to have on my desk (ready to be returned to the library) a copy of U. S. Submarines in World War II: An Illustrated History of the Pacific by Kimmett & Regis. There is a graphic on page 87 which describes the “Lethal Zone” as detonation within 25 feet of the sub’s hull. Within 50 feet is labelled “Serious Damage.”
It’s not clear, unfortunately, whether those radii apply to the Type 95 or Type 2 Japanese depth charges, both of which are briefly described in the same graphic. Oddly, the Type 95 is the older model, containing 242 pounds of explosive; the Type 2 holds 357 pounds, and also can apparently support a deeper detonation depth. Both could have their detonation depths set in 100-foot increments.
Here’s a conclusion one might draw from those numbers, which I wish the book addressed: If the detonation depth steps and the lethality radii described above are accurate, then it would seem to behoove a submarine being depth charged to seek a mis-matched depth (150 feet, 250 feet, etc.), as that would appear to significantly improve chances of survival - the only way a charge would get closer than 50 feet would be due to error of the detonation depth and/or the sub’s depth holding.* But the book doesn’t go into this - nor can I recall hearing it discussed anywhere else, either, so I suspect it wasn’t that simple in practice.