  The nuclear isomeric state $^{229m}$Th with an exceptionally low excitation
energy makes the $^{229}$Th isotope a crucial candidate for nuclear clocks and
many other applications. Efficient and controllable production of $^{229m}$Th
is essential and still remains a challenge. Here we report a novel approach for
efficient production of $^{229m}$Th by the excitation of $^{229}$Th to the
above-lying excited state at $29.19$ keV energy via the process of nuclear
excitation by electron capture (NEEC). We show theoretically that the
production rate of $^{229m}$Th per nucleus with accessible conditions can be
six orders of magnitude larger than the value experimentally demonstrated using
$29$-keV synchrotron radiation for this indirect excitation. With the efficient
production of $^{229m}$Th, our results identify scenarios, as well as the
characteristic NEEC signature with which NEEC events could be unambiguously
identified, for a clear experimental identification of the long-sought NEEC
phenomenon.
