Sometimes a bubble is hard to recognize until after it has burst.
Sometimes it is easy to see.

In 1637 a single tulip bulb in the Netherlands was traded for a 12 acre farm, or 10 times the annual income of a skilled tradesman.
That's clearly a bubble.

So is this.

Interest rates today are the lowest they’ve been at least since Stonehenge was under construction and the pharaohs of the First Dynasty ruled Egypt, Bank of America Merrill Lynch (BAML) said in a report this week.
In fact, according to another new report, this one from the Bank of Montreal, interest rates have fallen so far that some governments can actually make money by borrowing it — essentially destroying any argument for spending cuts.
“In this bizarre circumstance, let’s just say that the argument for spending restraint — let alone austerity — pretty much collapses,” Bank of Montreal chief economist Doug Porter wrote in a report Friday. “Public sector borrowing becomes a revenue-generating activity.”

Anytime you are using the expression "5,000 year high/low" it is a bubble by definition.
Anyone who says otherwise is either "talking their book" or a fool.

Ultra-low interest rates in Japan have completely wiped out the entire money market fund industry.
Persistently ultra-low interest rates will implode the global pension industry. Like money market funds, pension funds aren't built to survive zero interest rates for extended periods.

Need more proof? How about this.

The yield on Germany’s 10-year government bund, Europe’s benchmark security, fell below zero for the first time on record, as investors’ seemingly insatiable demand for haven assets created another bond-market milestone.
The nation joined Japan and Switzerland in having 10-year bond yields of less than zero.

Some of you may not understand the significance of one of the most important nations in the world going negative (NIRP).
Fortunately there is a precedent for this, and I have a chart that will make it more clear.

To put it more simply, the last time interest rates in Germany went to zero, it was followed by this:

Does that mean hyperinflation is coming? No. In fact I think deflation is much more likely. At least that is what the markets believe.

According to BAML’s “Longest Pictures” report, this is happening because of central banks all over the world that have been pushing down interest rates for years in order to fight the threat of deflation — a persistent decline in prices....
Central banks are battling that trend by lowering rates, which is supposed to spur more borrowing and spending, and therefore higher prices. But after nearly a decade of low and lower interest rates, and the threat of deflation still stalking developed economies, some are wondering whether the policy works at all.
It may have just caused the world to take on way too much debt.

Guess what happens to an economy with way too much debt? Debt deflation and eventually defaults and bankruptcies.

The Bank for International Settlements (BIS), a sort of “central bank of central banks,” has been warning for some time that the borrowing numbers aren’t adding up — the world has gone and borrowed too much.

This is the doing's of the global central banks, who have managed to convince the markets that "bad economic data is good news for markets" through the mechanism of massive financial asset purchases.

Those massive financial asset purchase programs, aka QE, have pushed interest yields on bonds to previously unthinkable lows.

Negative-yielding government debt has risen above $10tn for the first time, enveloping an increasingly large part of the financial markets after being fuelled by central bank stimulus and a voracious investor appetite for sovereign paper.
...
Negative rates have not been confined to the sovereign debt market. Corporate bonds with a negative yield have climbed to $380bn, according to Tradeweb.
“You have central banks which have gone through the looking glass and gone through . . . zero that many people thought was unbreakable,” said Sameer Samana, a strategist with Wells Fargo Investment Institute.

Central banks have been at DefCon1 for eight years now.

The balance sheet assets of the world's six major central banks hit a new all-time record, increasing to $16.9 trillion from $4.9 trillion 10 years ago, a 239 percent increase.
The balance sheet asset of the Fed, which has been leading the monetary policies of America, the world's largest economy, reached $4.495 trillion, equal to 25.41 percent of the country's combined GDP. This proportion was 6.17 percent in 2006.

So what? What's to stop them from continuing this forever?

Once again, Japan is the note of caution.

The total assets of the Bank of Japan (BOJ) reached 70 percent of the country's GDP in 2015 with $2.834 trillion, compared to 24.27 percent in 2006.

Japan's central bank has bought up so many of its bonds that liquidity in their bond market has dried up. It's a major reason why NIRP in Japan is a bigger disaster than anywhere else.
Secondly, like a snake swallowing its own tail, the amount of bonds that the Bank of Japan can even buy is shrinking.
Japan is not alone.

German lender Commerzbank AG estimates that almost two-thirds of all German sovereign debt outstanding now yields below minus-0.4%, which makes it ineligible for central-bank purchases.

Thus QE, like falling interest rates, has both diminishing returns and an upper limit.

Economists believed for at least five centuries, since the creation of modern economics, that negative interest rates were not possible.
You don't throw out the rule book without some amount of care.

Unlike 2007, this bubble isn't being led by the United States (although we're getting a lot of the excess cash/credit/debt).
Europe and Japan are leading the charge into unsustainable low interest rates, while China is leading the charge in excessive corporate debt and housing bubbles.